The regional synoptic process with moderatetoheavy rain appeared in the eastern Chaidamu basin on 7 to 10 July 2005 is analyzed by means of the averaged field data. It is closely correlated with the branch trough separated from the Balkhash low trough and the lowvortex shear in the lower/middle troposphere over the Chaidamu basin, as well as the southwest warm/wet air around the subt ropical high. The convergence in the middle troposphere, the maintenance of divergence in the upper troposphere and high humidity, the coupling of strong vorticity and stability columns and the maintenance of the upper/lower jet flow are the main causes of the moderate to heavy synoptic weather process.