The invention relates to a piecewise calculation method for a thermal power generating unit peak regulating energy consumption cost. According to the method, force outputting threshold values at different pitch peak stages of the thermal power generating unit are determined according to a nameplate parameter and a unit performance testing of the thermal power generating unit. The peak regulating process of the thermal and power generating unit is divided into three stages based on a running state and an energy consumption character of the thermal power generating unit. Aiming at differences of the running state at different pitch peak stages of the thermal power generating unit and from a point view of benefits of a heat-engine plant, the cost of loss, the additional cost of environment and the cost of oil investment in the thermal power generating unit are calculated on the basis of considering a coal fired energy consumption character, a piecewise function of the thermal power generating unit pitch peak energy consumption cost is constructed, and further a curve of the thermal power generating unit pitch peak energy consumption cost is obtained. The piecewise calculation method for the thermal power generating unit pitch peak energy consumption cost can provide a basis for economic operations of the heat-engine plant.