A structure of intra-cavity frequency doubling in a continuous-wave (CW) Yb-doped double-clad fiber laser is proposed. High conversion efficiency green laser can be obtained in theory due to the high pump power density in the cavity. Advances in compact structure of double-clad fiber laser also allow for the availability of laser integration. In this work, the optimized efficiency is achieved when the waist is located at the edge of MgO∶PPLN and the Rayleigh length is equal to twice the length of the crystal, and the lens′ location in the system was calculated. 10.5 mW green laser is obtained, and the second harmonic generation (SHG) efficiency of the system is 0.35%. Laser oscillation in the cavity is verified and it is indicated that the structure can achieve effective CW green output.