Abstract In a liquid injection heat pump, it is very essential to control the compressor discharge temperature without wet-compression problems at extreme outdoor conditions. The objective of this study was to optimize the injection-hole geometries of a liquid injection heat pump in order to prevent the risk of wet-compression while reducing compressor discharge temperature at overload cooling conditions. In this study, a simulation model for predicting the performance of a liquid injection heat pump was developed and validated. The optimum injection-hole geometries were determined to obtain the maximum multiplication ratio, which led to a lower instant injection mass flow rate in terms of R- and θ-directional positions. In addition, the injection-hole diameter was minimized to prevent wet-compression while obtaining the target injection mass flow rate. The discharge temperature of the optimized compressor was decreased by 9.2 °C over the baseline compressor while maintaining the same risk for wet-compression at the overload cooling test condition.