The invention belongs to the technical field of solar thermal power generation equipment, and discloses a combined solar thermal power generation system. The system comprises a parabolic trough collector subsystem, a heat exchanger subsystem, a Rankine cycle power generation subsystem and a dish power generation subsystem; the parabolic trough collector subsystem comprises a trough-type mirror field, a pump and a valve; the heat exchanger subsystem comprises a superheater, an evaporator and a preheater; the Rankine cycle power generation subsystem comprises a temperature-decreased pressure reducer, a steam turbine, an electric generator, a condenser, a condensate pump, a deaerator and a feedwater pump; and the dish power generation subsystem comprises a dish-type mirror field and a Stirling engine set. The system utilizes the heat released by the cold chamber of the Stirling engine by condensed fluid of the Rankine cycle. It provides an extra heat source for the Rankine cycle, which increases the power of the steam turbine and improves the solar to electric efficiency of the thermal power generation system.