Vapor injecting could effectively improve thermodynamic performance for heat pump with high temperature rise. The performance of medium/high temperature air-source heat pump using vapor injection need to be conducted. A simulation model of the vapor injection heat pump with an economizer is established to analyze influence of parameters (temperature, injection ratio) on cycle performance. Parameter optimization and comparative evaluation of three typical medium-and high-temperature working conditions are carried out under different cycle temperature spans. The results indicate that vapor injection could increase the heat rejection and COP of heat pump, but it has limited effect on reducing compressor exhaust temperature for isentropic working fluids. There exists an optimal branch evaporation temperature that maximizes the COP of the system. The optimal value is greatly influenced by the cycle temperature span and is less affected by the type of working fluid. The corresponding injection pressure is equivalent to the arithmetic square root of the cycle high and low pressures. Increasing the injection ratio is beneficial to improving cycle performance, but the maximum upper limit of the injection ratio is re-stricted by the heat transfer temperature difference of the economizer, and the type of working fluid and cycle temperature span have a significant impact on the maximum allowable injection ratio. The performance improvement effect of vapor injection on large cycle temperature span is more significant. Under condensing temperature of 60℃ and 90℃，COP is improved by 4% and 16% for R1234ze(E)，respectively.