Abstract: The paper aims to explore the causes and mechanism of snow storm and extract the technical indicators of snow storm forecast. The conventional meteor ological observational data and MICAPS data were used to carry out a diagnostic analysis of a snow storm process occurred on November 5-7, 2015 in northcentral Shanxi. The results showed that: (1) the snow storm occurred under a favorable background caused by the interactions of airflows separately from high and low latitude, and by the combined effect of the surface inverted trough and the backflow, the formation of the backflow was caused by the cold air which was divided from the high pressure and blew southward and stored in the northeast China plain; (2) the maintenance and superposition of low-level jet and super low-level jet were benefited to the abundant provision of water vapor for the snow storm area; the big vapor flux area was consistent with the position of the low-level jet, and the convergence area of moisture flux divergence had a very close relationship with snow storm area, which had a significant instruction for snow storm forecast; (3) the start time of snow was closely related to the convergence and ascending motion; when the dry and cold northeaster flew back to the northern Shanxi and converged with the warm and wet south low-level jet, the shear line and the dry line were formed and the snow was began; after that, the snow gradually expanded to the south with the move of shear line and the dry line southward. The shear line and the dry line was the trigger system of the snow storm process; the northeast cold air on the northern side of the shear line and the dry line played a role as a cold pad, which forced the warm moist air flow to climb along the cold pad, then ascending motion was strengthened and the snow storm was prompted.