The energy efficiency improvement of air conditioning systems is very important. The relevant equations of air-conditioning heat exchanger are established based on the basic thermophysical laws in this paper. While R22 is the working fluid, the heat transfer equations of condenser and evaporator in gas and liquid two-phase region were solved with two different diameters ϕ 9.52mm and ϕ 7mm. The influence of diameter on fluid and heat transfer performance of air-conditioning heat transfer is analyzed based on calculated results. It shows that decreasing the diameter of the heat exchanger tube will be beneficial for the optimal design of heat exchanger if the increasing degree of the average heat transfer coefficient is higher than that of the total pressure drop. The conclusions of this study could provide theoretical basis to optimize the design of domestic air-conditioning heat transfer.