When the multispectral remote sensor is working in the space, it will be affected by the contamination of chemical molecules, vibration during launch, radiation from space and etc. Its spectral characteristic will be degraded by time. As a result, bias will show in the physical measurement and cannot be rectifiec by radiometric calibration, which will affect the constancy of the data products. An on-orbit degradation assessment method is proposed, which is based on spectral targets. By establishing the degradation model of response function and using the nonlinear reflectance of spectral targets, quantitative assessment of the degradation can be achieved. The results of the simulation experiment and in-flight experiment verify the effectiveness of this method. The method has great significance in improving the accuracy of radiometric calibration, optimizing the camera design and other aspects of remote sensing.