Solid-gas thermochemical sorption heat storage has attracted considerable interest in recent years due to high energy density and the ability of long-term storage duration. Operation principle of thermochemical sorption heat storage system is based on reversible solid-gas chemical reaction by storing thermal energy in the form of chemical potential. The comparison of thermochemical adsorption and resorption heat storage cycle has been firstly made. Then, the thermochemical composite sorption heat storage cycle is presented for the effective utilization of the low-grade thermal energy. As a novel thermochemical sorption heat storage cycle, thermochemical composite sorption heat storage has integrated the merits of both thermochemical adsorption and resorption heat storage cycle. Furthermore, the performance of a thermochemical composite sorption heat storage system is analyzed by employing MnCl₂-SrCl₂-NH₃ working pair. Theoretical analysis showed that thermochemical composite sorption heat storage cycle is effective for integrating energy storage and energy upgrade of low-grade thermal energy. It can not only decrease the heat input temperature of external heat source during the heat storage stage, but also upgrade significantly the temperature level of stored thermal energy during the heat release stage. As a result, thermal energy temperature can be upgraded from 81 °C to 170 °C via thermochemical composite sorption heat storage cycle. Therefore, it is very beneficial to enlarge the application temperature range and realize the high-efficient utilization of low-grade thermal energy.