In the Large High Altitude Air Shower Observatory (LHAASO), the square kilometer array, with
5216 electromagnetic particle detectors (EDs) and 1188 muon detectors, is deployed to explore the
gamma-ray sources above 30 TeV with unprecedented sensitivity and to measure primary cosmic
rays in the energy range from 10 TeV to 100 PeV. The stability of detector performance is crucial
for the physical research of LHAASO. In this paper, we present an established offline calibration
technique for determining the crucial performance parameters of the EDs. This robust calibration
also offers an ideal method to monitor the performance of thousands of EDs during their science
exposure. According to the calibration results, the output charge of EDs has an average temperature
coefficient of -0.33%/◦C, and the detector relative time-offsets have no apparent correlation with
temperature. Using the relationship between the output charge and temperature (dominant), the
annual rate of performance degradation is studied after correcting the temperature effects.