Two channel elliptical crystal spectrometer (TCECS) is a very important diagnostic instrument for research on inertial confinement fusion (ICF) controlled by laser. Through time and space resolution detected simultaneously for X-ray, experimental condition of ICF can be judged correctly. There are many influencing factors for resolving ratio of TCECS, which are elliptical eccentricity, aperture width, light source not locating in the front focus of ellipse, the axis of detector does not pass through the back focus, aiming and alignment error. This paper analyzes main influencing factors for resolving ratio, using Matlab 6.1 software does numerical calculation, and presents ways to assure and increase resolving ratio of TCECS.