In virtual cathode oscillator (VIRCATOR) research, the radiated microwave pulse is coupled out and propagates along a waveguide. The radiated microwave power is an important parameter to evaluate the oscillator design and is a prerequisite parameter to calculate the energy transferring efficiency. Because the radiated microwaves are an ultra-short pulse of multi-frequency in multi propagating modes, the power measurement is not easily manipulated. In this paper, a broadband calorimeter design with considerations of pulsed microwaves is presented. The design includes its material or medium selection, structure optimization and calibration.