Based on the routine observations,an ageostrophic wet Q-vector diagnosis of a heavy rain process over the eastern Qinghai-Xizang Plateau during 18-19 July 2005 is made. The results show as follows:(1) The heavy rain occurred in the updraft area triggered by the convergence center of the wet Q-vector.The convergence center is narrow and strong during the heavy precipitation.(2)The superposition area of the center of the positive vor-ticity and the convergence center of the wet Q-vector at 700hPa is advantageous to the me- soscale system.It corresponds well to the heavy rain area.(3)The center of frontogenesis at 700hPa corresponds to the heavy rain area after 12 hours.After the instability energy is re-leased largely,the air mass has frontolysis,and the heavy rain decreases gradually.