Study on the intensity changes of the cosmic rays during thunderstorms is very important for understanding the acceleration mechanism of secondary charged particles caused by an atmospheric electric field. In this work, Monte Carlo simulations were performed with CORSIKA to study the intensity changes of positrons, electrons and photons in thunderstorms electric field. We found that the number of these secondary particles changes significantly. The variation amplitude is closely related to the strength and polarity of the electric field. A series fields were chosen in our simulations, ranging from -1000 V/cm to 1000 V/cm. The correlations of the intensity variations between positrons, electrons and photons were also discussed. The LHAASO (Large High Altitude Air Shower Observatory) project is located at Daocheng, Sichuan province, China, at an attitude of 4400 m above the sea level. As the main part of this project, the electromagnetic particle detector (ED) in the kilometer-square array (KM2A) and the water Cherenkov detector array (WCDA) are sensitive to the secondary positrons, electrons and photons in extensive air shower (EAS). Our simulation results may provide important information for giving reasonable explanations to the forthcoming experimental data of LHAASO.