A concave quartz window (concave lens) is installed at the front end of a cavity receiver to improve the uniformity of energy flow and gain advantage of the redistribution ability of the functional curved surface of an optical concave lens to the transmission of solar light. A combined ray-tracing method and genetic algorithm optimization design method are used to optimize the geometric size and installation position of the concave lens and improve the uniformity of the energy flow of the dish system. Twelve types of concave lenses are combined with four distinct quadric surfaces and three curved surfaces. The benefits and characteristics of concave lenses in improving the uniformity of energy flow of the dish system are analyzed in detail, and the effectiveness of the proposed method is verified. The results show that adding an optimized concave lens to the dish system can significantly improve the energy flow uniformity of the cavity receiver, reduce the local energy flow peak, and obtain excellent optical and energy efficiency. The optimized double cone lens showes a significant reduction of the non-uniformity factor from 0.59 to 0.11.