The leakage problem is very serious in Jiangxiong Reservoir, and the leakage occurs during its construction. With the lastest three times of grouting treatments, the water leakage is still large. According to the geological structure and engineering geology, the fact that the leakage water is from different parts is obtained considering the probing results of the detection ditch. In order to determine the leakage passages and water leakage amount, borehole temperature detection is used. Based on layer situation, the leakage position from 0+615 to 0+655 is considered to be plane leakage. The plane-leakage heat source can be treated as the superposition of line-leakage heat sources. Based on the theory of heat conduction and the energy conservation principle, a formula for the plane-leakage heat source model is established. Considering the difficulty of practical application, through a certain degree of approximation, a simplified method based on the plane heat source is proposed. It may provide a new method for complex leakage problems in engineering. The calculated results of leakage parameters of the leakage position from 0+615 to 0+655 agree with the measured values, indicating that the proposed model can be employed to analyze the plane leakage in engineering and that it may be useful for similar projects.