Thunderstorms, accompanying with lighting flashes, are frequent phenomena at high altitudes. The intensity and polarity of thunderstorm electric field will change dramatically. The charge structure in thundercloud is one of the hottest topics in atmospheric electricity. To know more about the thunderstorm effects on the variation of cosmic rays detected by ground-based experiments, the thunderstorm activity at LHAASO observatory and the charge distribution in thundercloud are studied in this work. The diurnal variations of thunderstorms are analyzed. We find the thunderstorms are most active in late afternoon to evening, even a few of them occurring at midnight. Based on disk model, the near-earth electric field is calculated with different charge structures in thundercloud.At the same time, using the electric field data from two ground observation stations, an inversion method is proposed to study the charge distribution in thundercloud. The evolution of several typical thunderstorm processes at LHAASO observatory are analyzed in detail. Our results will be helpful in understanding the acceleration mechanism of secondary cosmic rays caused by an atmospheric electric field,the cosmic ray variation detected by LHAASO during thunderstorms, and also be useful in the study of atmospheric physics.