The invention relates to the field of thermal power generation and discloses a double reheat thermodynamic system. The double reheat thermodynamic system comprises a steam turbine, a condenser, a by-pass turbine, a combination drive turbine, a high pressure heater; the steam turbine comprises an ultrahigh pressure cylinder, a high pressure cylinder, a medium pressure cylinder and a low pressure cylinder; an exhaust port of the ultrahigh pressure cylinder is respectively connected with the combination drive turbine and the high pressure heater; an extraction port is arranged on the combinationdrive turbine and the extraction port is connected with the high pressure heater; an exhaust port of the medium pressure cylinder is respectively connected with the low pressure cylinder and the by-pass turbine; and an exhaust port of the by-pass turbine is connected with the condenser. According to the thermodynamic system, the economy of an electric generating set can be greatly improved by thegradient utilization of energy; meanwhile, when the load of the electric generating set changes, it can be guaranteed that the high pressure cylinder, the medium pressure cylinder and the low pressurecylinder all work within a high efficiency working condition range, the station service power consumption rate can be reduced and the overall working performance of double reheat can be improved.