The improvement of wind speed forecast accuracy by modifying error can significantly improve wind power efficiency of wind farms, increase the stability of power grid connection, relieve the power grid pressure and reduce the economic cost. In order to select the reasonable modification method of wind speed in different wind farms, the forecasted error of wind speed by GS-WPFS were modified by using the Weibull, AVT and MOS methods at Heiyazi and Nanqiu wind farms of Gansu Province from 3 June to 31 August 2016. The results show that the average errors of forecasted wind speed were 2 to 3 m·s-1 before the modification, while those decreased to between 1 and 2 m·s-1 after the modification. By comparing, the average error of modified wind speed by MOS method was lowest, followed by Weibull and AVT methods. However, there were significant differences in modified results to forecasted wind speed with different grades by three methods. For Heiyazi wind farms, the best modification effect of forecasted wind speed with 0-4, 4-8 and 8 m·s-1 and above were AVT, MOS and Weibull methods, respectively. However, for Nanqiu wind farms, when the wind speed was 4 to 13 m·s-1, the modification effect of Weibull and the AVT methods were significantly better than that of MOS method.