To verify the precision and possible applicability of the large mass method (LMM) widely used in multiple-supported structures subjected to non-uniform base excitations, numerical simulations of a two-degrees-of-freedom (2-DOF) finite element model using the Rayleigh damping assumption are performed respectively according to the LMM and the relative motion method (RMM). Through comparisons with the RMM, the error origins and the applicability of the LMM are discussed. Then the improved LMM is presented herein based on the modification of ground motions considering the influences of Rayleigh damping coefficient α. It indicates that the LMM is not applicable to multi-support excitation analysis in the case of Rayleigh damping, which can cause significant errors. And the errors depend on the damping coefficient α. It’s also proved that the improved LMM is able to yield results that are identical to those of the RMM.