The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) has been operated for more than ten years. The onboard spectrophotometer channel-1 (SP1) was designed to detect the far-ultraviolet (FUV) emissions (N2 Lyman-Birge-Hopfield band) from space. It was found that most of the transient luminous events (TLEs), such as sprites, halos, elves, and gigantic jets, are usually accompanied by FUV emissions. The FUV emission could be utilized as an indicator for searching TLEs because the FUV emission of lightning is severely attenuated in the atmosphere during the propagation, though the FUV emission of TLEs is still detectable. Furthermore, the photon flux of elves FUV emission can also be used to estimate the peak current of their parent lightning. Therefore, the assessment of the sensitivity degradation of the SP1, which affects the observed intensity of FUV emissions, is important. Although ISUAL has a light-emitting diode (LED) which is preset as a standard light source, the emission wavelength range of the LED does not cover the FUV band. In this report, we established a method to determine the sensitivity degradation of the ISUAL Imager and the SP1. We found that the annual degradation rate of the ISUAL Imager and the SP1 are 8.4% and 9.7%, respectively.