A miniature spectrometer comprises an input port, a light sensor, a miniature diffraction optical grating, an optical grating accommodation slot, a cushion, and an affixing plate. The miniature spectrometer may further comprise a waveguide device, and the optical grating accommodation slot is positioned in a space defined by an opening of the waveguide device. The input port receives an optical signal which proceeds in the waveguide device. The miniature diffraction optical grating separates the optical signal into numerous spectral components to be projected onto the light sensor. The cushion is stacked on the miniature diffraction optical grating, with both disposed in the optical grating accommodation slot. The affixing plate is disposed on the waveguide device to apply a compressing force on the cushion to affix the miniature diffraction optical grating in the optical grating accommodation slot.