A partial differential equation (PDE) in the form of transient light intensity is derived for describing the nonlinear absorption of an ultra-short pulse laser in the isotropic dielectric media with only the third order susceptibility. For a Gaussian pulse laser, an analytical solution to PDE is obtained under some approximation. The related nonlinear transmittance depends on not only the peak intensity of the pulse laser, sample thickness and two-photon absorption coefficient, but also the frequency and the full width at half maximum (FWHM) of a laser pulse. The simulations of the experimental results of the materials such as Rhodamine show that the two-photon absorption coefficients of the media based on the approximate solution to PDE are generally lager than the ones based on the existing nonlinear transmittance, which are qualitatively the same as the conclusion given by fluorescence method. The results suggest that the active nonlinear transmittance may have some deviations due to the absence of the reasonable deals with the transient behavior of an ultra fast laser.