To meet the requirements of high spectral resolution and the weak target detection of the ocean remote sensing satellite, the high signal-to-noise ratio (SNR) must be realized in the narrow band and low illumination. By deriving the SNR formula of the imaging spectrometer, the evaluation model is established. By using the optical transmittance, the diffraction efficiency of the grating, the quantum efficiency of CCD, the dark current of CCD, the readout noise and the video noise, the SNR of the system in 5 nm spectra bandwidth is analyzed at ocean color spectral, center wavelength and typical radiance. At last, the system SNR is tested. The results show that the simulation results are correct, the SNR of the imaging spectrometer can achieve to 60 dB.