The transfer slab is generally replaced by equivalent cross-beam systems when the integral structure of a tall building is computed. In this study, partial analytical computation of the transfer slab with the Finite Element Method was done, the factors influencing the width of equivalent beam were found, and the width of the equivalent beam was obtained by reverse calculation according to the principle of equivalent rigidity. The conclusion was that the main influencing factors were the span and thickness of the slab, and that the ideal computing formula for the width of the equivalent beam was exponential fitness depending on whether there existed concentrated load or uniformly distributed load.