With the criteria of linear and nonlinear symmetric instability theory,the mechanism of the heavy snowstorm of January 17—18 1995(″95.1″)occurred in the northeast region of the Qinghai-Xizang Plateau was diagnosed by using the output data of a MM5 mesoscale numerical model with which the developing structure and the evolution of the ″95.1″ snowstorn were fairly well simulated.The results suggested that the positive σ² is corresponding to the development and movement of the snowstorm and the limited comparison lends some support to the hypothesis that non linear CSI can be a dominant formative mechanism of the ″95.1″ snowstorm,though more sophisticated studies are required.