Quasi-phase-matched second harmonic generation(SHG) in bulk periodically poled L iNbO3(PPLN) was studied , the coupled-wave equations for quasi-phase- matched S HG was accurately resolved, and the formula for calculating SHG conversion effic iency under focusing Gauss beams was given. The relations between focusing beams  waist and crystal length were analyzed. Furthermore, the SHG cavity was opti mized to obtain the maximum conversion efficiency.