Formulas for the transfer function and the output power gain are presented for an Er 3+/ Yb 3+ codoped series-cascaded double microring resonator (MRR). Around the pumping wavelength of 974 nm and the central signal wavelength of 1530 nm, the effects of the pump power, signal power, Er 3+/ Yb 3+ dopant concentrations and amplitude coupling ratios on the signal output spectrum are analyzed. The computed results show that the device can possess both the filtering and amplifying functions for the signal light. In the case of the amplitude coupling ratio between the microring and the channel being 0.17, and that between two microrings being 0.012, the device can produce more than 10.13 dB signal power gain.