In the Large High Altitude Air Shower Observatory (LHAASO), the square kilometer array, with 5249 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 energetic particles produced by extensive air showers can serve as a continuously available source for calibration of the numerous EDs over a large area. In this study, the detector untriggered probability is first proposed to estimate the particle density at different distances from the shower core and distinguish the characteristic single-particle signal detected by each ED. This method uses science data directly, and does not require prior knowledge of the cosmic-ray elemental composition or hadronic interaction model. Experimental results show that this self-calibration can be used to determine the number of particles detected by each ED with an accuracy better than 2% within a time scale of hours, which is adequate to meet the physics requirements of the LHAASO experiment. With this high efficiency and accuracy, this calibration also provides an ideal method to monitor the detector performance throughout an expected lifetime of > 10 years.
