On August 14，2018，Typhoon Yagi（2018）moved northward and impacted Shandong Province of China，resulting in widespread rainstorm and heavy rainstorm. The total rainfall caused by the typhoon in Shan‐ dong presents a round-shaped distribution. Specifically，on August 14，an outer spiral rainband appeared on the typhoon periphery in southeastern Shandong，bringing short-term heavy rainfall and local heavy rainstorms. Due to the relatively small scale of this rainband，both numerical forecasting models and forecasters face challenges in predicting its rainfall accurately. To study the mechanisms of the outer spiral rainbands of Typhoon Yagi，the characteristics and causes of the spiral rainbands are investigated in this study by using radar data and the observations from ground-based stations，radiosonde stations and aircraft. Numerical experiments are also conducted based on the Advanced Research WRF（Weather Research and Forecasting）model and its Hybrid-3DVAR （three-dimensional variational）data assimilation system. The model adopts 12 km and 4 km one-way nested grids，with 44 vertical layers. The initial ensemble perturbation fields are generated by using a stochastic perturbation method，and the Ensemble Transform Kalman Filter（ETKF）method is used for the bias correction of ensemble forecast，providing flow dependent background errors for the Hybrid-3DVAR assimilation module. Comparative experiments with and without the Aircraft Meteorological Data Relay（AMDAR）data assimilation are conducted by adopting 100% flow-dependent error covariance and by using a 45-minute assimilation time window. The results indicate that the outer spiral rainbands are formed by the merging and development of several linear mesoscale convective systems（MCSs）. The outer spiral rainbands exhibit distinct characteristics of the linear MCSs with leading stratiform precipitation，i. e. ，the linear MCSs consist of several convective cells with back-building convection. There are several stronger linear MCSs merging laterally into other linear MCSs. Broad stratiform echoes appear in the front（eastern part）of the linear MCS in its maturity stage，and the convection develops up to 10 km or more. There is a weak-echo transition zone between the strong convective line and the sub-strong stratiform echo region. Short-term heavy rainfall occurs along the linear MCS at the maturity stage. The water vapor of heavy rainfall mainly comes from the near-surface layer（below 850 hPa）around the typhoon，and the water vapor flux convergence is mainly concentrated near the wind field convergence line. Be‐ fore convection initiation，the middle and lower levels over Shandong are thermally unstable with high temperature and high humidity，and the wind rotates clockwise with height，which favor the development of convective systems. As the typhoon slowly moves northward，downward intrusion of cold air appears at 500 hPa. Below 900 hPa，on the southeast of the typhoon over central Shandong there are local convergence between southwesterly wind and southerly wind，and between southerly wind and southeasterly wind. The convergence-induced dynamic uplift triggers the release of unstable energy，stimulating several local linear MCSs. The MCSs develop north‐ ward along the steering flow. The linear MCSs merge and strengthen for several times，and finally the elongated spiral rainbands occur. During the convection lifetime，the updrafts are noticeably stronger than the downdrafts. At the mature stage of the convective systems，dry and cold downdrafts appear in the lower levels in the front of the MCS. Convective systems at the heights above 600 hPa move rapidly eastward with the upper-air steering flow，leading to the gradual weakening and dissipation of the linear MCS. Assimilation of AMDAR can improve the typhoon track and wind field forecasts of the WRF model，as well as the dynamical triggering mechanism of convective systems. Thus，the occurrence of spiral rainbands in the typhoon periphery could be accurately fore‐ casted. Furthermore，central Shandong is a mountainous region，so how does the topography influence the triggering and developing of convective systems？ What are the differences between typhoon outer spiral rainbands and the main body spiral rainbands？ What are the differences between outer spiral rainbands？ These issues de‐ serve further studies.