To obtain the variation of wind speed near ground at the southeastern coastal area of China and its influence factors during the landfall process of typhoons, wind speed samples at 10 m during the landing of two strong typhoons (Typhoon Matsa 0509 and Typhoon Khanun 0515) were recorded by anemometer towers located at the regions of Donghaitang in Zhejiang Province and Luchaogang in Shanghai. Wind speed conversion factors of different time intervals and their probability distribution were calculated based on the sampling wind speeds. It is illustrated that wind speed conversion factors of different time intervals follow generalized extreme value distribution. V_{3 s}/V_{10 min} follows Frechet Distribution, while V_{30 min}/V_{10 min} and V_{1 h}/V_{10 min} follow Weibull Distribution. There are 2 anemometer towers, one is adjacent to the paths of typhoons, while the other one is far away from them. It shows that in typhoons' condition for terrain A, values of wind speed conversion factor keep steady and the probability distribution functions of conversion factors hold similar distribution characteristics during the landing process of strong typhoons regardless of the paths of typhoons, variation of typhoon intensity, the distance between the anemometer tower and the typhoon center and the landfall state of the typhoon. Values for wind speed conversion factors for terrain A in the condition of typhoons based on the analysis of probability statistics provide the assurance of reliability for wind resistant design of structures.