The traditional slowly varying envelope method can′t be used for propagation of femtosecond pulse with duration of several optical oscillation periods. By simplifying the electric field's wave equation with unidirectional wave approximation and incorporating the material equation the propagation equation of electric field of such extremely short optical pulse can be attained. With this method the new propagation equation of extremely short optical pulse with arbitrary polarization state in isotropic medium has been presented. The interaction between two polarization parts has also been studied numerically.