A continuous-wave (cw) singly resonant optical parametric oscillator (OPO) with the wavelength of 3.81 μm is obtained by pumping the periodically poled MgOLiNbO3 (PPMgLN) crystal with a homemade Yb single-mode fiber laser at 1064 nm. The laser uses the all optical master oscillator power amplifier (MOPA) structure. The cw singly resonant OPO used the e→e+e phase matching in crystal. The PPMgLN is 50 mm long and the period is from 28.5 μm to 31.5 μm. The cavity is formed of two mirror linear strusture. Pumped by the 49 W linear polarized ligth field at 30 ℃, the idler light 3.81 μm is obtained. It′s maximan power can reach 4.25 W. The threshold value of the pumping laser power is 5 W, corresponding to the output efficiency of 31.1%. When the temperature of the PPMgLN is changed from 21 ℃ to 170 ℃, the wavelength of the output laser changes from 3.65 μm to 3.82 μm. And the corresponding spectrum of OPO is measured. The experiment results are different from the theoretical results when the temperature of the PPMgLN exceeds 110 ℃.