Abstract: Based on daily surface minimum air temperature data of 135 observational stations from 1961 to 2009 in Northwest China and NCEP/NCAR reanalysis data,authors analyzed the climatic variation characteristics of the last frost date by using the methods linear trend analysis,Mann-Kendall,Moving Texamination,Wavelet analysis,power spectrum and composite analysis methods.The results showed that there were large spatial difference of last frost end date in Northwest China,it was impacted by altitude that the cold period were long,so the area which last frost end latest was Qinghai plateau,and the secondly one was north Xinjiang-Hexi corridor-Ningxia and east Gansu-north Shanxi.The earliest area was south Xinjiang and south Gansu-south Shanxi.The end date advanced at the rate of 1.9d/10y,the stations occupied 87%of whole region.The significant stations occupied 45%,the most significant area was northwest Qinghai-east of south Xingjiang,reached 5-8d/10y.From middle time of 1990s,the last frost end early and had little inter annual variation,it had abrupt change in 1997 and had the remarkable 2.4 years and 2.7 year periods in recent 49 years,the last frost end date were advanced under the background of global warming.But in recent 10 years,the stations which had delayed trend were increased,this phenomenon worth pay attention.The late frost was impacted by the North Africa subtropical high and polar vortex,the former was negative correlation and the latter was positive correlation.The reason of late frost was delayed may be related to the polar vortex area increased and the North Africa subtropical high area reduced.The 500hPa composite analysis indicated,in late frost end early years,the northern Asian was prevalent zonal circulation,the East Asian trough was shallow,the area of Northwest China was controlled by the Mongolian high ridge,cooling weather was not easy to appear;in late frost end later years,the northern Asia was prevalent radial circulation,there was cold air which constantly slided downward from Northern Europe in Northwest China,it was easy to form strong cooling weather.

Key words: Late frost date, Climatic change, Circulation, Northwest China