The agricultural water market of China is not extremely perfect, and the current standard of irrigation water pricing is somewhat low. Along with the gradual realization of the modernization and rehabilitation of irrigation districts, the level of irrigation water supply services is improving and so how to price irrigation water becomes very important while the users of irrigation water, farmers' attitude and willingness are the foundations of water pricing. Taking Wu'an Irrigation District as a case study, the aims of this paper are estimation of the farmers' willingness to pay (WTP) for improving irrigation water supply services and identifying the factors affecting WTP. The results show that the farmers' WTP are distributed in the range of 375 similar to 825 RMB/ (hm(2) . a) and the mean WTP is 633 RMB/ (hm(2) . a). And four important factors including farmers' gross income, the proportion of plant production income, the duty condition and the water supply condition have significant influence on the farmers' WTP. By analyzing the relations among the standard of irrigation water pricing, the proportion of supporting farmers and the gross income of water rates, the reasonable water pricing scope is elicited, 525 similar to 600 RMB/ (hm(2) . a) in the future three years. Then, some reasons for the possible biases are inspected and some recommendations are put forward.