The Water Cerenkov Detector Array (WCDA) is a component of the Large High Altitude Air Shower Observatory (LHAASO), comprising 3,120 cells, each equipped with a large PMT (8" for pool1 and 20" for pool2 and pool3) and a small PMT (1.5" for pool1 and 3" for pool2 and pool3). The large PMTs are used to measure the time and charge of secondary particles produced by extensive air showers (EAS), while the small PMTs extend the measurement range of charge. As the performance of these PMTs varies from one to another and changes over time, it is crucial to measure their performance in real-time. In this report, we present a method that uses single muons from air showers itself to measure the charge efficiency of large PMT. To do so, we select shower muons that hit in the vicinity of small PMTs, whose position and direction can be predicted, to study the response of large PMTs. By simulating the charge that the muon would produce on the large PMTs in the same cell, based on its position and direction, we can calculate the charge efficiency of the large PMTs. We found that the measurement efficiency is consistent with the lab sample, and that the average charge spectra of large PMTs from simulations and data agree within a 10% uncertainty. Using this method, we obtained an average efficiency, and will study the efficiency of individual large PMTs later.