The errors of wavelength in elliptical crystal spectrometer are divided into two models:1) The target center is not on the focus of the ellipsoid, the target size can not be ignored, and the background scatters from any directons; 2) The X-ray is reflected from irregular crystal surface, which is not an ideal ellipsoid. The analysis of these two models are gvien and the relation between the wavelength errors and the aperture slit width are psented with digital calculation. The calculation data are agreeable to the experiments．