One severe convective weather event which is so rarely seen in arid areas, occurred in Karamay City of Xinjiang on 14 July 2012. The convective event lasted for more than an hour, accompanied by short time severe downpour and hail. The process of convection is studied with composite dataset including NCEP FNL analyses, automatic weather station observations, satellite cloud image, Doppler weather radar and simulations of mesoscale numerical model WRF. The results show that, in the background of large scale instable moist stratification and complex mountain terrain, horizontal temperature gradient and topographic convergence shear get enhanced on the lee side of mountain where convective weather develops easily. The numerical simulation results of WRF model indicate that the configuration of high and low level wind, vertical shear of horizontal wind and saddle type of vertical pseudo equivalent potential temperature provides power for the ascending motion, driving the W wind speed rapidly to the maximum 6.5 m·s-1 and top level of ascending up to 9 km. The thin layer of moist neutral stratification limits the growing of vertical ascending motion so that the area of ascending motion is narrow and small, causing the severe precipitation to happen only in a local area. Mountainous terrain plays an obvious and complicated role in developing strong convection. The convergence of ground wind is enhanced by orographic effect, and the nonlinear disturbance of airflow over mountain prompts the rapid development of severe convection.