The working principle of radiometer is introduced, and a principle prototype of 35 GHz bandwidth integrated direct detection radiometer is developed, in order to meet the demand for radiometer imaging system. The signal is amplified by a two-stage low noise amplifier which gains 40 dB, then directly detected by the detector, and finally amplified by video amplifier. The design and testing are performed separately to the detector, with detector voltage sensitivity at 75-100 GHz greater than 1 000 mV/mW, at 100-110 GHz greater than 500 mV/mW. A radiometer is completed. Response over 35 GHz band, integration time of 0.6 ms and temperature sensitivity of 0.45 K are achieved.