Nowadays, L+ band (1600-1650 nm) is widely used in optical communication, however, the traditional fiber amplifiers for L+ band can no longer meet the increasing demand, and it is crucial to select more suitable materials and optimize the peak power of the output spectrum. In this work, we determine the rate equation and power propagation equation corresponding to the three-energy-level structure by Er3+ doping phosphate glass, and select the appropriate pumping wavelength and power, and calculate the variation of amplified spontaneous emission spectra with the length of the fiber and the doping concentration using MATLAB programming. Finally, on this basis, genetic algorithm is used to optimize the fiber length and doping concentration, and the parameter values to maximize the peak power of the output spectrum are successfully obtained, and the performance is compared with that of the traversing parameter method, which provides a reference for the design and optimization of fiber amplifiers.