A computer-executable method is executed by a CPU as the following steps of: obtaining an interested range in relation to a target typhoon and obtaining historical typhoons within the interested range from a wind and typhoon database; obtaining shortest distances from the respective historical typhoons to the target typhoon within the interested range; choosing a target ground grid point and obtaining normalized extreme wind speeds of the respective historical typhoons corresponding to the target ground grid point; obtaining extreme wind speeds probable for the target ground grid point by calculation according to the normalized extreme wind speeds and the highest wind speed of the center of the target typhoon; and arranging the extreme wind speeds in descending order, arranging the shortest distances corresponding to the respective extreme wind speeds, and obtaining the occurrence probability of the extreme wind speeds at the target ground grid point according to a formula.