The invention discloses a multi-mode combined power cycle system based on transcritical CO2 and a method, and belongs to the field of aviation power propulsion. According to the multi-mode combined power cycle system based on transcritical CO2 and the method, a turbine engine, a ramjet and the supercritical CO2 Brayton cycle are coupled, so that a hypersonic aircraft can be switched in multiple modes, and therefore the purpose of wider-range work is achieved. The system comprises an air inlet channel (1), an inner duct (2), an outer duct (3), a precooler (4), a compressor (5), a main combustion chamber (6), an air turbine (7), an afterburner (8), a spray pipe (9), a CO2 turbine (12), a condenser (13), a pump (14), a liquid CO2 tank (10), a liquid oxygen tank (15), a first switch (11) and a second switch (16). According to the combined power cycle, transcritical CO2 closed heat energy gradient recovery Brayton cycle is adopted in a super-precooling flight mode, and liquid oxygen is utilized for condensation, so that the energy utilization of the system is maximized; and in addition, supercritical CO2 is adopted as a pre-cooling working medium to replace traditional helium, the system structure is more compact, the thrust is larger, and the wider-range working requirement can be rapidly met.