We analyse the relationship of conversion efficiency with the inter-crystal phase shift by the heuristic theory and propose a novel configuration of two cascaded nonlinear crystals for the second-harmonic generation with the polarization modulation. With this configuration, 70% exteal doubling efffciency is obtained, which is, to the best of our knowledge, the highest conversion efficiency with LBO crystal exteal frequency doubling. This configuration provides a simple and effective method to improve the second harmonic conversion efficiency.