The interference between adjacent diffraction orders is unavoidable in the two-dimensional spectrogram of an echelle grating spectrometer in the long wavelength band due to the non-uniform dispersion of prisms. To avoid this drawback and simultaneously take full advantage of the detector's imaging plane, we design a small-size echelle grating spectrometer with a divided spectral coverage is designed The relationship between the detector and the parameters of the above two is established via the detailed analysis of the dispersion principle of echelle grating and prism. The design approach of the echelle spectrometer with a divided spectral coverage is proposed with the design idea of double-slit interval and by the way of double-slit switching. With this design approach, the wavelength range of 165-800 nm for the system is divided into 165-230 nm and 210-800 nm, the focus length is set as 200 mm, and the two-dimensional spectrograms at the two bands are collected, respectively. The ray tracing simulation to the optical system is carried out and the results show that the true spectrum resolution at 200 nm is 0.015 nm, which meets the design requirements.