This paper introduces the development of solar simulation technology, gives design specifications of solar simulators, and then discusses the design and improvement of optical systems of solar simulators in detail. Several design methods of the condenser system are proposed, especially, a compound high-order aspheric surface condenser system and a non-coaxial ellipsoidal mirror are presented to raise the concentration efficiency of condensers and improve the irradiance distribution of the second focal plane. As the design of optical integrators is based on the Gaussian optical theory and its optimization is guided by the aberration theory, it presents an effective empirical formula for optimum design of optical integrators. This paper also briefly analyzes the design of collimating systems and the transmittances of filters. Finally, the current problems existed in the process of development are summarized and several improved ideas are put forward.