Adsorption desalination is an emerging method to address the shortage of fresh water. To achieve its energy saving potentials, a renewable low-temperature heat source is the key. This paper focuses on the area optimization of solar collectors in combined solar heating systems that supply heat for adsorption desalination, aiming at achieving the maximum economic benefits during the lifespan of a project. The optimum tilt angle was discussed and the corresponding daily radiation data were calculated with HDKR (Hay, Davies, Klucher, Reindl) model. The area optimization function was proposed with the unit costs of fresh water as the optimization objective. A reference area calculated according to annual solar radiation was employed to describe the optimization results. Then the influences of the price of auxiliary energy sources, the performance ratio of adsorption desalination, and the heat loss of solar heating on the optimization results were studied with an actual adsorption desalination project in Tianjin. It has been found that the optimal area is positively associated with the auxiliary energy price, however approximately inversely proportional to the performance ratio of desalination and the energy efficiency of solar heating. The unit costs of producing fresh water can be reduced up to 20% after collector area optimization when the auxiliary energy is expensive.. Besides, the overall costs of utilizing solar heating to supply heat for adsorption desalination is around 0.03-0.04 CNY/MJ which is lower than conventional energy sources. We also tested the optimization results with an altered precision of radiation data and different meteorological conditions. Findings in this paper proved the advantages of incorporating solar heating in adsorption desalination and provided instructions for designing solar adsorption desalination projects.