An eigenvector method(EM)is given to calculate the optical field in laser with flowing saturable gain media. Flowing staturable gain media is equivalent to a thin gain sheet just near inside the big mirror of laser resonators cavity. And transit matrix of EM is obtained on the basis of this model. The calculated results show that the flowing saturable gain media has a negative effect on the optical field distribution. It causes the optical field distribution inclines along the direction of gas flowing. The results are in good agreement with other methods.