Based on the principle of measurement of celetial body's brightness, the signal-noise ratio (SNR) of photon-multiplier-tube (PMT), common charge-coupled-device (CCD) and electron-multiplying CCD (EMCCD) was deduced, the performance of these three devices had been compared in theory and experiment. The results showed that the performance of EMCCD was the best in all situations. When the radiation was fairly weak, the dominating factor which had an effect on detection ability was readout noise of CCD, so the performance of PMT was better than CCD's. If the radiation is fairly strong, the limits detection ability was quantum efficiency, then the performance of common CCD was better than PMT's. Finally, sum of squared error and signal-noise ratio (SNR) were experimentally validated, it showed that experimental value was consistent with theoretical value.