The invention discloses a method and device for determining wind speed probability distribution and a method for evaluating the power of a wind power system. The method includes the steps of receiving an average value and a standard deviation, which are counted in advance, of the wind speeds of an area to be tested in n hours, calculating a probability density function of the wind speeds in each hour with a normal distribution function according to the average value and the standard deviation, converting the probability density functions, obtained through calculation, of the wind speeds in the n hours into probability density functions of the wind speeds in the n hours under standard normal distribution, calculating wind speed probability distribution in each hour with the probability density functions of the wind speeds in the n hours under the standard normal distribution, and determining average wind speed probability distribution in the n hours. According to the method, the wind speed probability distribution is solved with a normal distribution function, the process only needs first-class wind speed parameters and does not need second-class wind speed parameters, and therefore wind speed probability distribution can be calculated based on existing stored data.