Pipe resistance coefficients (PRCs) identification of water distribution systems (WDSs) is a process of adjusting the PRCs in hydraulic model of WDSs to make its predictions consisting with measurements. Because the number of monitoring sensors is limited in practice, the identification of PRCs of WDSs is an under-determined optimization problem. Existing methods tend to use a parametric method of pipe grouping to convert the under-determined problem to over-determined, and then solve it using GA or other stochastic searching algorithms. This paper presents a prior information based algorithm for PRCs identification of WDSs. In the proposed method, the PRCs are estimated previously according to prior information of pipe material and pipe-age, and then used as pseudo observations introduced into objective function to convert the under-determined optimization problem to over-determined one, and the Gauss Newton algorithm is utilized to solve it. Compared to existing method, the proposed algorithm avoids the non-uniqueness problem of pipe grouping; in addition, the analytic formula of Jacobian matrix of PRC is deduced for searching vector construction, which improves the calculation efficiency of parameter identification. A simple network was used to illustrate the calculation of Jacobian matrix and the construction of search vector, and a larger network was utilized to validate the practicability of the method.