The introduction of internal heat exchanger can improve system performance in vapor compression system. Compared with R134a, refrigerant R1234yf has a lower global warming potential, and its environmental effect is more obvious. The energy balance equation and the exergy balance equation were established for R1234yf heat pump system. The effects of evaporation temperature, condensation temperature and superheat degree on the system coefficient of performance COP, heating capacity, exhaust temperature and system efficiency were analyzed and compared with R134a system. The results show that the performance of the two systems is similar, but the COP and the heating capacity of the R134a system are 1.39%-4.06% and 0.53%-4.08% higher than that of the R1234yf heat pump system, respectively, and the system efficiency is 2.03%-3.27% higher than that of the R1234yf heat pump system. As the degree of superheat increases, the difference between the COP and the efficiency of the two systems are reducing.