Abstract: A simplified algorithm for internal blast wave propagation in multi-room buildings was developed based on forward ray tracing, aiming to rapidly predict the propagation range of blast waves in buildings partitioned by masonry infill walls. The algorithm emitted ray bundles from the explosion source into space and generated uniformly distributed rays by performing triangulation on the spherical blast wavefront, ensuring that the ray density adequately represented the energy distribution of the incident wave. The algorithm simulated both the direct propagation of blast waves and their reflection off wall surfaces, and employed a time-stepping method to track the propagation paths of each ray within rooms and their interactions with wall boundaries. To address the blocking effect of infill walls on blast wave propagation, the walls were discretized into regularly arranged masonry units using a grid model with dimensions consistent with the actual masonry units. Based on the bond failure criterion of individual masonry units and the overall shear failure criterion of the wall, the opening state of the wall was determined at each time step; when a masonry unit reached the failure condition, it was removed from the grid, allowing rays to penetrate through the damaged region into adjacent rooms, thus enabling the simulation of continuous blast propagation across multiple rooms. The proposed model was validated through comprehensive comparisons with experimental data and finite element simulations under both single-room and multi-room configurations, focusing on key metrics including wall surface overpressure time histories and overall damage ranges. Good agreement is achieved across all test cases, confirming the model's capability in rapid assessment of internal blast loads in buildings. It should be noted that the model is specifically developed for buildings partitioned by masonry infill walls; for scenarios involving rigid walls or structural components with openings such as doors and windows, the current formulation requires further refinement and additional treatment of diffraction effects.