In this paper, the crossover effect of the dimensionality in the disordered layer system is studied by using the post-CPA method for the vertex. The effective dimensionality of the system is determined by the interlayer coupling t. It is shown that Anderson transition will occur at the critical value tc. Boltzmann conductivity, the dynamical conductivity corresponding to the maximum crossed diagrams, the d. c. conductivity and the localization length near the critical point are calculated.