Abstract: In the report, based on the similarity between the electrostatic field and the law of universal gravitational field, the concept of the loop theorem of universal gravitational field was established, and the analogy was performed to obtain the loop theorem expression of the static gravitational field. Then, the gravitational potential energy was introduced. Due to the density of the sun is generally considered as a celestial body with symmetrical spherical distribution, after establishing a piecewise density distribution function, the gravitational potential energy inside the sun will be given. With the help of the sun numerical solution model, MATHEMATICA software was used to obtain the temperature in the core of the Sun. Our results are consistent with 1.5×10⁷K estimated by hydrostatic equilibrium of the sun’s interior, which is to be achieved for sustaining a thermonuclear burn.