In order to solve the problem of low radiation uniformity of a solar simulator, a variable coefficient ellipsoid condenser system was established according to the working principle of solar simulators. Firstly, the high-order equation is obtained by Taylor expansion of the traditional ellipsoid equation, and an ellipsoid equation with variable coefficients is obtained by evolution. Then, the optimal variable coefficient ellipsoid equation is obtained by adjusting the coefficients of the variable coefficient ellipsoid equation. Then, MATLAB is used to simulate the energy at the second focal planes of the variable coefficient ellipsoid and the traditional ellipsoid, and the two are compared. The LIGHTTOOLS software is used to simulate the optical systems of the traditional ellipsoid and the variable coefficient ellipsoidal condenser. The simulation results show that the uniformity of the second focal plane of the variable coefficient ellipsoid is higher than that of the traditional ellipsoid. The radiation uniformity of the variable coefficient ellipsoid is 3.91% higher than that of the traditional ellipsoid, which basically meets the requirements of high radiation uniformity of solar simulators.