Abstract: In this paper, the nonlinear motion equation for the PBL over equatorial zone is solved numerically with different distributions of the pressure with latitude and the motion charactristics of the PBL over the equatorical zone are obtained. The results show the important role played by advection and the solution indicates that corresponding to ITCZ, near the equartor there exists a concentrated zone where the vertical velocity at the top of the PBL is great both when there is a low pressure zone near the equator and the pressure decreases with the increase of latitude monotonically, this overcomes the difficulty of the critical latitude mechanism which neglects the large nonlinear advective terms.