The crosstalk between two adjacent signal channels of concave grating demultiplexer is composed of the transverse mutual coupling efficiency between two output waveguides and the response efficiency of output waveguide for the central wavelength of adjacent signal channels. Under the condition of weak coupling approximation, based on the mutual coupling coefficient between two nonparallel rectangular waveguides which derived from the method of field distribution analysis, the mutual coupling characteristic between two identical structure output waveguides in concave grating demultiplexer is discussed, and the relationship between the crosstalk and the basic structure parameters of the output waveguides is clarified. As the spectral response efficiency which is derived from the diffraction factor of grating facet, emission factor of input waveguide and receive factor of output waveguide is employed, the relationship between the spectral response characteristic of the signal channel and the basic structure parameters of the Rowland circle structure concave grating demultiplexer is clarified. These results will provide theoretical support for designing the basic structure parameters of concave grating demultiplexer.