This letter studies a wireless powered sensor network (WPSN) where the senor nodes harvest energy from the power beacons by wireless power transfer (WPT). In order to improve the energy transfer efficiency of WPT, a WPT scheme with energy beamforming is presented to power the sensor network. Moreover, for the sake of maximizing energy efficiency (EE), we present a resource assignment optimization problem for this scheme, which is nonconvex because of the fractional form of the objective function. So we develop a low complexity algorithm to resolve this problem. Compared to other traditional algorithms (e.g., Dinkelbach [1] ) for solving this type of problem, whose objective function is of fractional form, it works with lower time complexity. Experimental results indicate the superiority of the proposed scheme.