Abstract: According to the frost index defined by the minimum air temperature ≤0℃,the climatic change of the anomalous first frost date,last frost date,frostless period and frost days were analyzed in this paper,using the daily minimum temperature of 9 stations at main agricultural area of Tibet from 1961 to 2010 and modern statistical diagnosis methods,such as linear trend analysis,accumulative anomaly,signal noise ratio and rescaled range analysis (R/S analysis).The results showed that，(1)in recent 50 years,the first frost date appeared late,the last frost date ended early,so that the frostless period was extended significantly in the part stations,while decrease trend of the frost days was detected with a rate of 1.9-9.6d/10y in all stations at main agricultural area of Tibet.(2)From 1980s to 2000s,because the first frost date appeared late and the last frost date ended early made frostless period prolong,and the frost days had a decreasing tendency,especially in the 2000s.(3)In addition,it was found that the frequency of anomalous early first frost date was 1 to 5 times at main agricultural area of Tibet,especially the maximum in Bome, meanwhile the frequency of anomalous late ending frost date was 1 to 6 times,the most in Bome and occurred in the 1960s.Frequency of anomalous short frostless period was 1 to 7 times,it was the most in Bome. Furthermore,the frequency of anomalous less frost days was 2 to 10 times and occurred in the 2000s,and the anomalous more frost days occurred in the 1960s with a frequency of 1 to 7 times.(4)It was found with abrupt change test that frost days in Lhasa,last frost date,frostless period and frost days in Tesdang,frostless period and frost days in Nagarze had abrupt change,which happened respectively in 1992,1991 and 1998.(5)The results of R/S analysis showed that changes of last frost date had the persistence with a Hurst index of larger than 0.5 in the part stations,which indicates that it will assume continuous earlier in future,especially in Tesdang.Also the Hurst index of frost days was larger than 0.5 in all stations,and it will assume continuous decrease in future.

Key words: Frost days, Hurst index, Anomalous, Climate abrupt, Main agricultural area of Tibet