By means of the conventional observation data and the improved wet 〖WTHX〗Q〖WTBZ〗 vector （〖WTHX〗Q〖WTBZ〗q）， a continued torrential rain process on the east side of the Qingzang Plateau between 18 and 19 July 2005 is diagnosed. The convergence field of 〖WTHX〗Q〖WTBZ〗q on 700 hPa indicated the region of the heavy rain， and the convergence center corresponded to the precipitation center. By dividing the improved ageostrophic wet 〖WTHX〗Q〖WTBZ〗 vector into two parts： parallel and perpendicular to isentropes， further research is made on the event. The results show that the largescale and mesoscale systems had different forcing of vertical motion on different layers. The superposition of two ascending motions produced the heavy rain. During the first phase of the continual heavy rain， the largescale ascending motion on 700 hPa was primary， while the mesoscale ascending motion was dominant in the lower and middle layers， and they caused the strong convection to develop and deep convection to form，which can reflect the area of the heavy rain more precisely. During the second phase of the heavy rain， the mesoscale ascending motion forcing was dominant. With the weakening of the heavy rain， the proportion of the largescale ascending motion increased， and the mesoscale decreased.