The relationship between thermal resistance ratio ζ(the ratio of thermal resistance between two sides of the tube wall) and load rate, according to heat transfer theory, is derived from simplifying the heat transfer model of heat exchanger. Considering the different running time of heat exchanger under variable load, the operation heat is proposed. Then the range of thermal resistance ratio for evaporator and condenser is determined and the law of thermal resistance is validated by analyzing the example. As a result, the relationship between rib effect coefficient and the thermal resistance ratio is established. Furthermore, value analysis of rib effect coefficient is analyzed. Results of this study show that the heat transfer performance of evaporator and condenser maximizes when ζ=1 at the partial load rate of (0.25, 0.50). Meanwhile, rib effect coefficient on the water side is less than that of the refrigerant side for evaporator and condenser.