The invention discloses a supercritical CO2 and ammonia combined cycle system and a power generation system. The supercritical CO2 and ammonia combined cycle system comprises a supercritical CO2 rankine cycle power generation system, an ammonia rankine cycle power generation system and a lithium bromide absorption refrigerator; and according to the system, the CO2 working medium of the supercritical CO2 rankine cycle power generation system is heated by means of a medium-high-temperature sensible heat source, supercritical CO2 rankine cycle turbine exhaust waste heat is used for driving an ammonia rankine cycle, and the supercritical CO2 rankine cycle turbine exhaust waste heat and ammonia rankine cycle turbine exhaust steam jointly drive the lithium bromide absorption refrigerator, and the cold flow generated by the refrigerator is used for completely condensing the CO2 working medium. The combined cycle power generation system utilizes self cycle waste heat to drive the lithium bromide absorption refrigerator, the cryogenic cold flow is provided for the condensing process in the supercritical CO2 rankine cycle, and stable operation of the system is guaranteed.