Starting from propagation law of partially coherent light beam,the expression for the spectrum of partially coherent light diffracted by a knife-edge in the far field was derived. Base on this expression,the spectra in the diffraction field were investigated theoretically. It is shown that the spectrum in the diffraction field changes with the location of the observation point,and for a fixed observation point,the spectrum varies with the spatial coherence of the partially coherent light beam and the ratio of transmission.