A singly resonant optical parametric oscillator (SRO) is built up based on periodically poled lithium niobate (PPLN) crystal pumped by a self-made high power and continuous-wave (CW) single-frequency 1.06 μm laser. A broadly tunable and high power CW single-frequency laser source in the infrared is obtained and the signal and idler can be tuned from 1.45 μm to 1.79 μm and from 2.62 μm to 3.99 μm by controlling the polarization period and temperature of PPLN crystal, respectively. The measured maximum output powers of the signal (1.52 μm) and idler (3.53 μm) are 5.1 W and 2.1 W, respectively, at a pump power of 15.5 W. The optical conversion efficient of the SRO is 46.5%. When the SRO is free running, the power fluctuation of the signal and idler are less than ±2.77% and ±2.79% for a given 4 h, respectively, and the frequency drift of the signal is less than ±45 MHz for the given 4 h.