With the global emphasis on energy conservation and environmental protection, solid wastes such as mine tailings, fly ash, slag, and beneficiation waste have increasingly been incorporated into the production of construction materials. As a result, the activity of radionuclides (such as 226Ra, 232Th, 40 K) in building materials increases, which significantly affects the radiation dose in indoor environments. This study uses the Monte Carlo method to model the gamma radiation distribution within indoor spaces. The model is based on the standard room geometry from the European Commission’s RP-112 effective dose calculation scheme. Photon sources with characteristic gamma emission energies corresponding to the decay chains of the uranium, thorium, and potassium isotopes were incorporated. Detectors were placed at various positions within the room to measure the photon flux at different locations. The simulation revealed the distribution of radiation flux across the space, with gamma radiation flux significantly higher near the walls, illustrating a typical boundary effect. This analysis indicates that the absorbed dose is the lowest at the center of the room and is the highest at the corners of the walls. When the distance to the walls is within 40% of the total distance from the center to the wall, the radioactivity increases significantly. The sensitivity of wall density and thickness to radioactivity is also analyzed. This study provides a theoretical basis and data support for understanding the radiation dose distribution in the indoor environment, which is helpful to improve the health protection of residential and commercial buildings, especially in areas with high natural radioactivity.