The South China Cathaysia block is located in the southeast margin of the Eurasian plate, and is an important mineral and geothermal resource area in China. After several tectonic events, granites of different tectonic stages are widely distributed in the area. Granitoids play a special role in the dynamics and evolution of the Earth and its thermal state, and the heat generated by their radioactive decay is one of the main heat sources on Earth. The high radioactive heat-producing hot dry rock resources are mainly distributed in South China, and the decay heat of radioactive elements is the main heat source, and the lithology is mainly granite. Based on the statistics of granite data in South China, this paper analyzes the heat generation rate of different granite types in different areas of South China, and summarizes the radioactive heat generation characteristics of granite in the study area and the relationship between them and terrestrial heat flow, which has reference value for understanding the main controlling factors of the geothermal field and the macro background of geothermal resource distribution in the study area. Based on the geochemical analysis of 1933 granite samples in South China, the following results are obtained: 1) The heat generation rate of granite ranges from 0.40 to 17.45 μW/m³, with an average heat generation rate of 4.56±2.66 μW/m³, and there is a good correspondence between them according to the distribution of ground heat flow and heat generation rate; 2) The contribution rates of radiothermic elements U, Th and K were 49.45%, 40.16% and 10.39%, respectively. At the same time, the concentrations of U, Th and Th/U varied greatly, and there was no obvious correlation with age; 3) There are differences in the heat generation rate of the three types of granite(Type I、A and S). The average heat generation rate of type I granite (3.86±2.43 μW/m³ ) is significantly lower than the average heat generation rate of type A and type S granite, which are 5.55±2.91 μW/m³ and 5.0±2.58 μW/m³, respectively; 4) A Monte Carlo method is used to calculate that the average heat yield of the Yanshanian granite in South China is 99.01×105 GJ/a(1 GJ/a=109 J/a), which is converted into standard coal 3.38×105 t/a, and that of Indosinian granite is 63.13×105 GJ/a, which is converted into standard coal 2.15×105 t/a; 5) The calculation of two crustal models shows that granite plays an important role in the contribution of earth heat flow in South China. In the area covered by sedimentary strata, the contribution of Earth’s radioactive heat generation to surface heat flow is 29.13 mW/m², accounting for 41.61% of the total heat flow value. In the area of intrusive rock exposure, the contribution of crustal radioactive heat generation to surface heat flow is 43.85 mW/m², accounting for 51.76% of the total heat flow value.