Silicon photomultiplier (SiPM) has been increasingly used in detectors of space telescopes. As a critical parameter of SiPM, dark current could be affected by many factors, such as temperature, overvoltage, and radiation damage. However, how the dark current of SiPM evolves in long-term under different space environments has not been systematically studied yet. SiPM is utilized in a series of GECAM instruments operating in different orbits and conditions, which provides a great opportunity to study this problem. Here, we present the first results on the SiPM dark current long-term (up to 5 yr) evolution of GECAM instruments, including GECAM-A, GECAM-B, and GECAM-C. We find that, while the short-term variation of SiPM dark current is primarily caused by the temperature fluctuation, the long-term evolution is predominantly determined by the accumulated radiation dose. Based on the GECAM design, we developed a model to describe the long-term evolution of the SiPM dark current, and all the long-term dark current evolution of GECAM-A/B/C could be well fitted by our model. Based on the comparison between GECAM instruments, it is evident that the SiPM dark current growth rate is independent of the working status of the SiPM but dependent on the radiation level, which is closely related to the altitude of orbit (500–600 km). We also find that the growth of the dark current can explain the decrease in energy gain of the detector. These results provide important guidelines for the performance study and prediction of GECAM instruments as well as the design of SiPM-based detectors of future missions.