Abstract: By using sounding data， surface observation and 1°×1°NCEP reanalysis data，diagnostic analysis and numerical simulation have been performed for a heavy rainfall on August 14，2015， in warm region of western plain in Sichuan.The results show that the subtropical high strengthened and moved northward，the south wind in the lower troposphere came by.The East Asia trough moved southward，led the low-level northerly winds into Sichuan Basin.The northerly and southerly winds converged， which resulted in the heavy rain.The south wind provided moisture， enlarged the potential pseudo-equivalent temperature in the layer near the surface which made convective instability intensified，and there was large vertical ascending velocity， all that was advantagous to heavy rainfall.By mesoscale filtering， There was convergence in flow field in Chengdu and Leshan，which corresponded to the beginning and southward moving of the rainstorm.The low-level wind convergence was the dynamic lifting mechanism of rainstorm in Chengdu.The numerical simulation found that the convergence of northerly and southerly winds at 700hPa was triggering mechanism of rainstorm in Meishan.Less moisture and weaker lifting in Leshan than that in Chend or Meishan resuilted in less rainfall in Leshan.