Based on the ageostrophic dry Q vector（QD）partitioning method，combining with the ERA5 reanalysis data and the China Meteorological Administration（CMA）tropical cyclone best track data，the causes of off‐ shore northward tropical cyclone（TC）gales in the coastal areas of Shanghai from 1981 to 2022 are diagnosed and analyzed， The results show that：（1）the stronger the wind force，the stronger the vertical upward motion， the lower the height of the strong updraft center and the thicker the vertical upward motion layer，which means the TC wind is closely related to the vertical motion within its circulation；（2）the range and intensity of the QD vector divergence convergence area are correspond to the vertical upward motion zone during the gale process. The vertical upward motion center coincides with the QD vector divergence convergence center. The stronger the vertical updraft is，the stronger the convergence intensity of the QD vector divergence，which indicates that there is a good correspondence between the vertical upward motion in the area affected by the TC gales and the convergence zone of the QD vector divergence；（3）the results of the QD vector partitioning method show that during the period of TC gale，the convergence intensity of QD s and QD n vector divergence increases with the wind scale. For the same wind level，the convergence intensity of the QD n vector divergence is stronger than that of the QD s vector divergence. Furthermore，the higher the wind scale is，the more distinct the difference between the convergence intensity of the QD n and QD s vector divergence. In details，the different convergence intensity in the scale 10 wind region can be larger than the scale 7 wind region by two orders of magnitude，suggesting that the forcing of the mesoscale weather system is the main cause of the TC gales. The results of this study provide valuable guidance and have significant practical application value for the forecasting of TC gales.