The wet bulb temperature among key meteorological parameters of final heat sink of inland nuclear power is determined as air wet bulb temperature. According to daily maximum air wet bulb temperature of hottest month for last 10a and for las t 30a, and yearly maximum air wet bulb temperature for last 30a from Tongshan me teorological station near the Dafan Nuclear Power Plant, three kinds of curves i ncluding logarithm, Gumbel and polynomials curves are used to fit the distributi on of the probability with maximum air wet bulb temperature and to calculate the possible value once for different periods. The result suggests that yearly maxi mum air wet bulb temperature for more than 30a and Gumbel curve fitting are suit able for calculating of parameter,although polynomial curve fitting is best, a maximum value can’t be overpass that limits its further application. Accordin g to the yearly maximum fitted by Gumbel curve, the final extreme valueonce for 50a, 100a, 500a, 1000a are 30.3℃, 30.5℃, 31.1℃ and 31.3℃ respectiv ely for the Dafan nuclear power plant.