Dielectric metasurfaces composed of arrays of subwavelength structures are capable of completely manipulating the phase, amplitude, and polarisation of light. In this Letter, a dielectric metasurface grating imitating the blazing behaviour of saw‐tooth gratings for imaging spectrometer is proposed. The authors present a method for designing the metasurface blazed grating by extending the effective medium theory to the two‐dimensional pattern. The designed metasurface blazed grating consists of periodical unit cells in both orthogonal directions and there contains seven sub‐periods within one unit cell. Duty cycles of each sub‐period in two orthogonal directions are independent, so that the effective index of both TE and TM polarisations can be manipulated simultaneously, and the grating's polarisation property can be optimised. Simulation results show that, for normal incident light, diffraction efficiencies of TE and TM polarisations at reference wavelength 0.7 μm are 79.2 and 79.3%, respectively. Within wavelength range from 0.6 to 0.8 μm, diffraction efficiencies are above 70% and the maximum degree of polarisation is 2.8%. It shows high efficiency and polarisation‐independent diffraction characteristics, and it may find various applications in optical frequencies, especially for quantitative imaging spectrometers.