The transcritical CO2 heat pump is highly efficient and environmentally friendly, making it a promising technology for water heating. This study investigates the startup characteristics of a transcritical CO2 heat pump water heater, focusing on the effects of inlet water temperature, flow rate, expansion valve opening, and startup strategy on the heating performance. An optimized startup strategy was proposed to improve startup efficiency and reduce energy consumption. Results show that lower inlet water temperatures and smaller expansion valve openings shorten the startup time. A comparison of cold and hot startup modes reveals that the hot startup method can reduce the stabilization times by over 30%, with significant reductions in power consumption.