A diagnostic analysis was made of the circulation background, dynamical and thermal and water vapor conditions for a heavy rain event on the 1 April 2004 over the eastern Gansu Province, aiming to find out useful forecasting hints for the sort of heavy rainfall. The results show that the main influencing factors include: temperature increased obviously 48 and 24 hours before raining; surface humid-static energy increased rapidly; there existed strong cold air activity, remarkable mid- and lower-level convection, profound positive vorticity, lower-level low-vortex convergence, and upper-level strong divergence at the end.