Many existing optimal solar-based hot water system design methods do not quantitatively consider the impact of energy mismatching, which leads to the overestimation of system performance and misleads the optimal design of the system. For addressing such mismatching problems, a solar-based hot water system optimization design method based on real-time energy balancing and co-adaptive matchmaking is proposed. It consists of three main parts: (1) establish a simplified solar hot water system model based on the real-time energy balance to simulate the dynamic energy performance of the system; (2) propose a co-adaptive matchmaking optimization method to minimize the energy mismatch between the supply and demand sides of the system; and (3) identify the scheme with the minimum life-cycle total cost as the optimum design. A multiple-function solar water heating system for the domestic hot water supply, space heating and air conditioning of a kindergarten building in Beijing was optimized based on the proposed optimization method. It was found that the energy mismatch degree between the supply and demand sides can be greatly reduced and leading to a maximum life-cycle saving cost up to 20.13% in the concerned case.