Based on the conventional observational data, automatic observational data of 123 stations in Shandong Province, and NCEP reanalysis data, the causes of a late spring extreme snowstorm on 19 and 20 April 2013 in Shandong Province are analyzed comprehensively, focusing on the temperature condition. The results show that the previous low temperature is the climate background, and the return flow synoptic pattern, the low level southwesterly jet, and the shear line in the north of Shandong are direct causes. The holding and convergence of water vapor were both happened in the layer between 700 and 500 hPa. The coupled reaction between high and low level jets and the positive vorticity column corresponding to the zonal shear line in the troposphere provided dynamic conditions for the snowstorm. The snowstorm of return flow weather occurred in the atmosphere with convective stability, and there was energy frontogenesis of θse over the snowstorm area in the mid low layer of the troposphere. Before the occurring of the snowstorm, the cold air pool accumulated by the return flow northeasterly wind blocked by the eastern foot of Taihang Mountain was the background of sharply falling surface temperature. The weak descending motion of the cold air mass between 700 and 500 hPa leaded to a funnel shaped 0 ℃ isotherm extending downward, resulted in the sharp falling of surface temperature and the conversion of precipitation phase from rain to snow in the snowstorm zone. So the zone with funnel shaped 0 ℃ isotherm extending downward can be regarded as the indicator of the conversion of precipitation phase from rain to snow and the occurrence of snowstorm in the future.