In this paper,a diagnostic analysis on a case of flash heavy rain occurred in the south part of Changchun on 20 July 1991 is conducted Results show (1) Before the occurrence of this flash heavy rain,the existence of θse-gradient reinforcement area,convective instability area and its tendency, Q-vector convergence area and greatly increasing of vapour content provide advantageous environmental conditions for the occurrence of the heavy rain (2) The energy transformation between the large-scale and meso-scale motions by non-linear interactions processes was a main physical mechanism for the formation of the flash heavy rain.