Large high altitude air shower observatory (LHAASO) is a complex of extensive air shower (EAS) detector arrays, located on the Mt. Haizi (29°21' N, 100°08' E) at an altitude of 4410 m a. s. l., Daocheng, Sichuan Province, China. The information about primary cosmic rays can be obtained by using data from secondary particles measured at LHAASO, with photons make up the majority among these secondary particles. During thunderstorms, the atmospheric electric field can affect secondary charged particles (mainly positrons and electrons), thus changing the information of photons on the ground. In this work, Monte Carlo simulations are performed to investigate the effects of near-ground thunderstorm electric fields on cosmic ray secondary photons at LHAASO. A simple model with a vertical and uniform atmospheric electric field in a layer of atmosphere is used in our simulations. During thunderstorms, the number and energy of photons are found to significantly change and strongly depend on the electric field strength. In a field of –1000 V/cm (below the threshold of the relativistic runaway electron avalanche (RREA) process), the number of photons is increased by 23%. Also, the spectrum of photons softens, and the increased number of photons with energy less than 2 MeV exceeds 29%. In an electric field of –1700 V/cm (above the threshold of the RREA process), the number of photons experiences exponential growth, with an increase of 279%. The spectrum of photons becomes softer than that at –1000 V/cm, and the increased number with energy less than 2 MeV is more than 361%. It is consistent with the theory of RREA. For these phenomena of photons at LHAASO, the main factor is that the number of positrons and electrons are increased due to the acceleration of negative electric field on electrons, with increase of 65% in –1000 V/cm and 992% in –1700 V/cm, and the spectrum of positrons and electrons soften. Newborn free positrons/electrons may undergo bremsstrahlung and deposit part of their energy into photons, causing the change of number and energy of photons to follow roughly the same pattern as positrons and electrons. The simulation results can provide the information for understanding the variations of the data detected by LHAASO during thunderstorms and the acceleration mechanisms of secondary charged particles caused by an atmospheric electric field.