In the framework of Keldysh-Faisal-Reiss (KFR) theory, with taking the coupling bet-ween the atomic potential and the applied light fields into account, the original KFR theory is improved and a new expression of photoionization probability of atoms is obtained.From this new expression, photoelectron spectra of H-atom ATI by strong linear-polarized laser in the short-pulse regime are got. This calulated photoelectron spectra are rather similar to the experimental ones. A new explanation of the experimental spectra is given.