A numerical model of intracavity second harmonic generation（SHG） is constructed considering the effect of phase mismatching and birefringent walk-off in nonlinear crystals for optically pumped semiconductor vertical-external-cavity surface-emitting laser（OPS-VECSEL）. The influences of the crystal length and the pump power on the SHG output power for some nonlinear crystals, such as BBO, LBO and PPLN, are studied in detail. The results are in good agreement with the experimental data.