The second-order approximate solution of P-doped cascaded Raman fiber lasers were obtained by nonlinear method. The differential equations depicting the distribution of pump and Stokes radiations along Raman fiber were simplified to the algebraic equations, and then the approximate solution under different input pump powers by second-order nonlinear method and the threshold expressions was obtained. The second-order solution agrees well with the numerical simulation. On the one hand, the second-order method was more accurate than the linear method and it could provide better guidance to the experimental design in regard to P-doped cascaded Raman fiber lasers for dual-wavelength outputs. On the other hand, the method effectively improves the numerical computational speed, and is superior to general numerical methods.