A spectrometer comprising a waveguide module, a diffractive component, and a light sensor is provided. The waveguide module has a first reflective surface, a second reflective surface opposite to the first reflective surface, and a light channel between the first reflective surface and the second reflective surface. The diffractive component has a diffractive surface and a plurality of strip-shaped diffractive structures located on the diffractive surface. The sharpness of the profile of the strip-shaped diffractive structures on a first side of the diffractive surface is greater than that on a second side of the diffractive surface. When viewed along a direction perpendicular to the second reflective surface, the first side of the diffractive surface is positioned between the first reflective surface and the second reflective surface with a distance away from the second reflective surface. A method for assembling the spectrometer and an assembling system are also provided.