An eye-safe laser source at 1. 57 μm with a maximum 64% energy conversion efficiency is demonstrated by use of a noncritically phase-matched (θ = 90℃, = 0°) KTP optical parametric oscillator pumped by a 1. 064 μm Nd: YAG laser. The effects of the noncritical phase-matching method on the walkoff angle and the acceptance angle of KTP crystal are analyzed and the operation properties such as threshold, efficiency and output beam quality of the OPO are discussed.