The threshold and stable operation of a continuous-wave singly resonant optical parametric oscillator (SRO) are closely related with the roundtrip loss of the resonant signal travelling in the resonant cavity. Under the plane-wave approximation, a theoretical model considering the linear absorption of the nonlinear crystal and the nonlinear loss introduced by intracavity-frequency-doubled (IFD) is proposed to describe the output characteristics of continuous-wave IFD-SRO. The analytical solution is also given. The theoretical calculation results are in good agreement with the experimental results. According to these results, the output characteristics of the 532 nm green laser pumped IFD-SRO with resonant signal at 780 nm and the 1064 nm infrared laser pumped IFD-SRO with resonant signal at 1560 nm are predicted. The model is simple and easy to calculate. It provides a guideline for optimizing the design of continuous-wave IFD-SRO.