Based on the daily average temperature data of the 15 weather stations in Hexi area in 1955—2009 and using the methods of climate trending rate, 5 days running mean temperature, Inverse Distance Weighted interpolation, Mann-Kendall abrupt change test and slide t detected method, the average beginning dates and length of four seasons have been analyzed in Hexi area of Gansu province in the same period. The main results are obtained as follows. (1) The season start dates of spring, summer and autumn become earlier, of winter become later, of summer have the most obvious change in recent 55 years. And this trend is the most evident in the beginning of the 21st century when the global warming is significant. (2) This trend has different spatial distribution in Hexi area. The season start dates of spring are more evident in advance in the east than in the middle and west, of summer and autumn are more evident in the middle than in the east and west and the delay of winter is more evident in the east than in the middle and west. (3) In the last 55 years, the averaged beginning dates of summer and autumn were advanced for 9 d and 1.6 d, respectively, and those of spring and winter were delayed for 7.1 d and 7.7 d respectively, and only that of the winter was changing significantly. The averaged lengths of four seasons are 68 d, 51 d, 53 d and 197 d, respectively. (4) The methods of T checking and Mann-Kendall were employed to analyze mutation of start dates. The abrupt change of start dates happened in 1997 in spring and summer while that of autumn appeared in 1993. The abrupt change of winter happened in 1977.