The dynamic process of nonlinear absorption in a semiconductor medium caused by nonlinear optical pulse propagation in the condition of nonresonance was studied.The threshold intensity formula was deduced in the limit of small absorption,which describes the relative size of two photon absorption and free carrier absorption.The effect of the sample thickness on the threshold intensity was analyzed.The main origin of the great disparity in the measured values of two photon absorption coefficient of GaAs was explained.