Solid desiccant cooling (SDC) systems are alternatives to conventional vapor compression refrigeration (VCR) systems for indoor air conditioning. Also, passive radiative sky cooling (PRSC) has been shown as an effective electricity-free technology for cooling. PRSC technology has also been incorporated in some air conditioning systems, which were demonstrated to improve system performance considerably. However, the use of PRSC technology in SDC systems has not yet been reported. In this work, a new SDC system was introduced, featuring the use of the PRSC component, i.e., radiative cooling panels, for cooling the supply air to improve the system performance. Mathematical models were developed for each component in the system, after which the models were combined to establish the simulation model of the whole system. The models were validated by comparing with the results in other papers. The performance of the conventional and the newly introduced hybrid SDC systems were evaluated with the established simulation models under the typical hot and humid weather condition in Hong Kong. The performance improvement by adding the PRSC component in the hybrid SDC system was analysed by comparing with the performance of the conventional SDC system. It is shown that the use of the PRSC component can further improve the cooling performance of the SDC system by lowering the air temperature by 3 °C approximately. Therefore, the new hybrid SDC systems with the PRSC component provide a more energy efficient method for indoor air conditioning than conventional SDC systems, especially in hot and humid regions, like Hong Kong.