The observation method is analyzed and the performances parameters of corresponding spectrometer are obtained based on the characteristic spectrums of the major elements in the underwater environment. The telescope and the imaging spectrometer are separately designed and coupled together. The telescope adopted double Gauss refractive system and realized optimization of the telecentric design with few optical materials. The imaging spectrometer based on the Dyson mounting also performed excellently with the analysis of optical path and the added lenses. The final design obtained the excellent performances that the field of view is 28°, the F number is 3, the spectral resolution is 3.5 nm, and the spatial resolution is 1 mrad in the waveband of 350 ~700 nm. The prototype verified the design theory.