The ISUAL experiment has successfully carried out global surveys of transient luminous event (TLE) and lightning since 2004. So far more than 11,000 TLEs and 90,000 lightning have been recorded using three sensors including an intensified imager, a spectrophotometer and an array photometer. The brightness of these TLEs and lightning in different band-passes can be derived based on the pre-launch calibration data. But the degradation of these instruments, especially the photon or electron multiplying devices driven by high voltage power supply, have not been precisely evaluated and reported to the science community in the period of the ISUAL mission. Without an accurate assessment of the degradation, the absolute photometry can not be achieved. In fact, routine performance measurements are done every month or season to monitor the degradation of ISUAL sensors and the instrument parameters are adjusted accordingly and periodically based on the reading changes of the stimulators which are installed inside sensors and are assumed to be a constant light source. The results show that the sensitivities of all sensors deteriorate gradually. Analyses of the long-term trending of the ISUAL sensors including the performance of the phosphor and the micro-channel-plate of the intensified imager, the sensitivity of spectrophotometer and the array photometer are presented. These results are not only useful for taking the degradation of the instrument into account for properly analyzed scientific results, but also important to perform a forecast of the ISUAL extended mission beginning from 2010.