An eye-safe laser pulse with 3 ns pulse duration,10 mJ output energy and 1.568 μm central wavelength was generated from a intracavity KTP optical parametric oscillator.OPO was formed by putting a noncritically phase-matched KTP crystal inside a passively Q-switched Nd∶YAG laser pumped by a pulsed Xe lamp.When the cavity length of the optical parametric oscillator were respectively 3 cm,6 cm and 8 cm,the pulse duration of signal laser were respectively 6 ns,4 ns and 3 ns.The double pulses were observed when the Xe electrical input energy was 26 J.The influence of the different Nd∶YAG laser resonator structure and parameter on the signal pulse were compared experimentally and analyzed theoretically.