To examine the annealing effect of Single Event Upsets (SEU), a single device from a Low Earth Orbit (LEO) satellite running in sun-synchronous orbit for more than nine years was selected as a sample. The working temperature from satellite telemetry data was investigated, and details of the SEU nadir and time were discussed in relation to the geomagnetic field, F10.7, neutrons monitoring data, etc. By averaging the SEU time intervals, a model was developed to estimate the annealing characteristics, and this model was analyzed using telemetry data. The results of this analysis indicate that the SEU nadir occurred above the South Atlantic Abnormal area more than 67% of the time, more than 16% over the polar area, and less than 17% over the remaining areas. Moreover, the SEUs have the highest probability of occurrence (more than 38%) in August, September, October, and December. However, the probabilities are equal when one group of months spans April to September and the other group contains the remaining months. Furthermore, the SEU frequency is influenced by cosmic rays in the long-term when the sun is relatively quiet and by the sun in the short-term when the sun is relatively active. The annealing estimation shows that the mean time interval between SEUs is about 4.57 d, the annealing zero value is close to 1.56×10-13 d with an annealing factor of 7.94×10-15 d-1, and the degradation zero value is nearly 24.34 d with a factor of 0.12 d-1. The estimation results indicate that the annealing effect is approximately negligible.