A broadband Fast Fourier Transform(FFT) algorithm radiometer is designed in order to detect the absorption spectrum of the atmospheric water. The design principle of the radiometer, the system structural design, the project design of the receiver and the spectrometer are introduced in detail. The main technical indexes of the orthogonal receiver and spectrometer are analyzed. This system can be used to detect the frequency of 22.235 GHz, and its designed bandwidth can reach 500 MHz with the spectral resolution of 60 kHz.