Based on the application requirements of differential absorption lidar for atmospheric CO2 concentration detection, a single-resonant KTP crystal optical parametric oscillator, pumped by a 1 064 nm single-longitudinal-mode laser, was built with stable ring-cavity and mode matching design, and nanosecond laser pulse output at 2.05 μm wavelength was achieved with high slope conversion efficiency. In the stable ring cavity with a bow-tie configuration, two KTP crystals with type II phase-match condition cut were placed in walk-off-compensated arrangement. When the pump pulse energy reached 11 mJ at a repetition rate of 20 Hz, 2.4 mJ of laser pulse energy output at 2.05 μm signal wavelength was obtained with pulse width of around 24 ns, and the corresponding slope efficiency was 53%. The 2.05 μm signal laser beam quality factor in x and y directions were 1.3 and 1.2 respectively.