Based on the theory of Fabry-Perot cavity, the optical diode effects of an asymmetric photonic crystal with a Kerr nonlinear defect layer are investigated. In order to effectively improve the anisotropic transmission and insure transmittance of forward incident light not too low, some methods may be used such as increasing the frequency shifting of incident light, enhancing properly the asymmetry at both sides of the defect layer, and making the incident light intensity near the low-value in the range of corresponding variations. The theoretical analysis is supported by the numerical simulation results.