This paper reports researches on the property of laser traveling in a thick medium and photoionization of laser atom interaction. The equation of three step photoionization for describing the field and the density matrix as well as the atomic population was derived. The ionization probability evolution with laser parameters is calculated in different cases. It was found that the laser transfer in a long distance causes evolution of laser parameters. Laser power, laser pulse shape, pulse width and time stepout of three laser bunches are the main factors of affecting the photoinization probability.