Second harmonic generation (SHG)- linear dichroism (SHG- LD) is an important method for the investigation of monolayer chirality at air/water interface. The chirality SHG response may originate from the contributions of electric dipole transitions or from contributions of both electric dipole transitions and magnetic dipole transitions. The distinction between these two cases is a prerequisite when studying the formation mechanism of the interfacial supramolecular chiraliy. The numerical simulations of the S-polarized SHG intensity as a function of the incident polarization angle are presented and compared for the two cases. The influences on the chirality SHG responses from the second-order nonlinear susceptibility tensors which account for the contributions of electric dipole transitions and magnetic dipole transitions are also analyzed, respectively. In addition, the significance of theoretical simulations to the analysis of experimental data is verified by the SHG- LD experiments on the supramolecular chirality of the porphyrin molecules at air/water interface.