A single resonant intracavity KTP optical parametric oscillator (OPO) is constituted by placing a non-critical phase-matched KTP crystal in a flashlamp pumped and passive Q-switched Nd∶YAG laser with an unstable resonator. The wavelength tuning property of output signal laser is studied and the tunable laser pulses of 1.57～1.60 μm are obtained. The experimental results show that the output energy of signal laser at 1.57 μm decreases, and the pulse duration changes to some extent, with the increase of the cavity length of the optical parametric oscillator. When the pump energy is larger, the energy conversion efficiency goes to saturation with the increasing of pump energy, and then drops gradually. And a reasonable interpretation is given theoretically. When the cavity length is 5 cm and the pulse duration of pump laser at 1.06 μm is 30 ns, the pulse duration and output energy of signal laser at 1.57 μm are 2.5 ns and 21.3 mJ respectively. The pulse duration of signal laser at 1.57 μm is only 1/12 of that of the pump laser at 1.06 μm, and the electro-optical energy conversion efficiency is 0.128%.