The utility model belongs to a microwave power measurement system, specifically discloses a megawatt grade impulse microwave power measurement system based on calorimetric method, wherein an outlet temperature pickup is arranged on the stream outlet of a water pipe and an electric heater is arranged in the water pipe; the electric heater is connected with a rated power electricity supply; a microwave detecting device is arranged on the inlet of a microwave power absorbing device; the water runs through the inwall water pipe of the power absorbing device and an inlet temperature pickup is arranged on the stream inlet of the water pipe; an NTC thermal resistance R3 of the inlet temperature pickup is connected with a equilibrium resistance R1, while an NTC thermal resistance R4 of the outlet temperature pickup is connected with a equilibrium resistance R2; the NTC thermal resistance R3, the NTC thermal resistance R4, the equilibrium resistance R1 and the equilibrium resistance R2 form a Winston bridge and the Winston bridge is connected with a differential amplifying circuit. The utility model can measure the multiplex pulse high-power microwave power in millimetric wave band and the measuring range can be one kilowatt up to two or three megawatt.