Compared with other forms of spectrometer, Offner spectrometer with a concentric structure is offering a lot of advantages, such as a compact structure, better imaging quality, lower distortion, and so on, convex grating on the second mirror is the key-component in it. The efficiency of an optical system plays an important role in application, but the low diffraction efficiency of Laminar grating or sinusoidal grating leads to a disadvantage for using, to solve this problem, blazed grating with much higher diffraction efficiency is a good answer. In application, blaze wavelength of the grating should be chosen in the spectral range with the lowest responsivity for the detector, so a proper blaze angle and theoretical prediction on affects by tolerance are important in designing the convex blazed grating in an Offner spectrometer. Based on the process of designing an Offner spectrometer, this paper deduced an expression for the angle of incidence in YZ-plane on convex grating in Offner spectrometer, then got the corresponding blaze angle, and analyzed affects on blaze wavelength brought by any kinds of tolerance, showed how to make the blaze wavelength work precisely for application, a practical example is also given.