The modes configuration of the electromagnetic field of the signal light and the pump light within rib Er-doped Al_2O_3 waveguide amplifiers were analyzed using the finite element method. The population distribution of the five-levels system for Er 3+ with 980 nm pump was solved using its rate equations. The relationship between the net gain of the waveguide and the signal power, and net gain characteristics of two or three rib Er-doped Al_2O_3 waveguide amplifiers in series were numerically simulated. The results show that there are two kinds of net gain losses. In the series system, the better total amplification ability can be obtained when the waveguide amplifiers with low-concentration is taken as a preamplifier than it is taken as a post-amplifier.