Abstract: Most of plates in building floor structures are irregular continous plates.The practical simplified calculation methods for this kind of plate are very useful.However, some simplified calculation method are being used wrongfully.To use wrongfully simplified calculation method can cause a relative error such as 20 percent or much more.Some inner laws of bending moments in the multi-span continous plates are discussed.For the veritable state of a loaded plate, one homologous state can be found, and the bending moments in this homologous state are the safe approximations of bending moments in the veritable state.The accuracy of safe approximations is discussed.The computation method of safe approximations is put forward.It is emphasized that one must note qualitative analysis in mechanical computation.