A KTP crystal extra-cavity single-resonant optical parametric oscillator (OPO) pumped by 1064 nm single-frequency laser is reported, and the nanosecond laser pulse output with wavelength of 2.05 μm is obtained. In the plane-parallel cavity, two type II phase-matched KTP crystals are placed in walk-off-compensated mode. When the pump single pulse energy reaches 5 mJ at repetition frequency of 400 Hz, a 2.05 μm signal laser output with single pulse energy of 0.9 mJ is obtained, whose pulse width is about 3.7 ns, the corresponding conversion efficiency is about 18% from pump light to signal light, and the beam quality factor M2 are 2.08, 3.03 in x, y directions, respectively.