Convex blazed grating is one of the key elements in hyperspectral imaging spectrometers. Generally, the effective wavelength coverage of blazed grating is not broad enough to meet the need of imaging spectrometer for grating′s diffraction efficiency. To extend the observation band of imaging spectrometer, in which the convex blazed grating was designed and optimized. A case study of 0.4-2.5 μm band Offner type imaging spectrometer, single-order and dual-order dispersive structures of convex grating were researched. Partitioned blazed grating and dual-angle blazed grating were used to improve the diffraction efficiency in the range of a broadband. The groove shape of two types of dual-blazed gratings with different dispersive structures was optimally designed, and the diffraction efficiency was calculated by the method of scalar theory and finite element analysis. According to the signal-to-noise ratio(SNR) of the instrument, different types of grating were proposed to meet different requirements of imaging spectrometer.