Based on simulation model of the sewage source heat pump system (SSHPS), weigh the influence of coefficient of performance under different work conditions, the ACOP was defined and as the optimization function, with the heat exchange areas of the evaporator, condenser and the sewage-midwater heat exchanger as the optimization variables. The optimization mathematical models for the on-off control scheme were set up to calculate the ACOP to study the optimal design and areas matching between the evaporator, condenser and sewage-midwater heat exchanger. Comparison with the initial design parameters and different optimization designs with HSPF, SEER and EER, the ACOP optimization is nearer to the objective energy conservation effect.