For the two-dimensional simulation of transversal seepage of the lock chamber, in order to take the effect of the width of lock chamber into account, the resistance coefficient model of inner section has been built based on the improved resistance coefficient method. By the local model of the ABAQUS software, this study changed the ratio of the foundation depth to the impervious depth and the ratio of the foundation depth to the outflow width, to calculate the discharge coefficient, the resistance coefficient and partition coefficient, based on the infiltration capacity, and then charts of different coefficients have been drawn. The result shows that the narrower the chamber width is, the larger the impervious depth will be, and the effect on the lateral outflow width will be more pronounced. For the lock of a lower level, narrower chamber width and larger water head, the impact of the chamber width should be taken full account to prevent the under estimation of the exit section slope, which is not conducive to the seepage stability. In this paper, the accuracy of the conventional resistance coefficient method has been improved, and the principle can be applied to similar projects, e. g. the effect of outlet width on the infiltration of a foundation pit.