A novel effectiveness evaluation method was proposed based on the Johnson criteria and minimum resolvable contrast to evaluate effectiveness. In this process， target and background contrast， atmospheric transmission， detectors， and human eyes， are considered comprehensively. The noise equivalent contrast is introduced to measure the noise level of the detector， and the detection efficiency is measured by the joint probability of contrast and resolution， which can quantitatively and intuitively evaluate the detection ability of space-based optical imaging system. Verification experiments are carried out based on aerial and GF-2 satellite images. In aerial images， the recognition probability of car is 46%， the identification probability of aircraft is 73%， and the NIIRS level is 4.23； in GF-2 satellite images， the recognition probability of truck is 67%， the detection probability of small ship is 63%， and the NIIRS level is 4.53. The results show that the probability calculated by our method is basically consistent with the subjective judgment of human eyes， which is basically consistent with the NIIRS. The results prove the effectiveness of the method. Our method is of great significance to the design of space-based imaging systems and the evaluation of the on-orbit satellite detection capability.