A method was put forward to eliminate spectral line bending (Smile) of Prism-Grating-Prism(PGP) spectrometer by reducing the vertex angle of the first prism and combining with off-axis slit and the distortion of convergent lens. The method could be used to retain the Bragg diffraction and coaxial characteristic of the PGP spectrometer and eliminate spectral line bending and spectral Keystone of the spectrometer. The Smile of the traditional PGP model and the improved PGP model with the same technical indexes were compared. In order to verify the feasibility of the method, a spectrometer with a spectral resolution of 2.8 nm was designed. The root mean square (RMS) of the spot radius was less than 8 μm, both imaging quality and design index requirements could be satisfied. The Smile and Keystone of the system were 1.50 μm and 3.52 μm respectively, which were both less than 0.2 pixel. The design result indicates that the Smile and Keystone of the PGP spectrometer can be effectively eliminated by reducing the vertex Angle of the prism to eliminate the Smile of the center wavelength, using the slit off axis to retain coaxial characteristic of the PGP spectrometer, using the distortion of convergent lens to eliminate the Smile and Keystone of the remaining wavelength.