The Gaussian beam theory has been developed to describe the power characteristics of intracavity continuous-wave singly resonant optical parametric oscillator (ICCWSRO). The second-order nonlinear interaction in the resonant cavity of laser is treated as the output loss of laser. The nonlinear reflectivity of the laser and the power characteristics of the signal and idler waves are obtained, based on the solution for the equations describing the power characteristics of laser. The stand-wave cavity structure, the properties of the four-level, end-pumped laser gain medium, the mean-field approximation of second-order nonlinear interaction, the diffractive effect of the interacting waves are all considered in the derivations. If the diffractive effect in the laser gain medium is neglected and only the first-order approximation is retained, the nonlinear reflectivity of laser is obtained, based on the solution for the transcendental equation. In the numerical calculation, the Nd∶YVO4 is taken as the laser gain medium, and the periodically poled LiNbO3 (PPLN) is taken as the second-order nonlinear crystal.