As a powerful tool for simulating bi-directional reflectance and radiative transfer (RT) in complex canopies, the radiosity graphics model (RGM) suffers from a reduced runtime speed or even crashes when facing considerable computation load of the view factor for fine-grained simulation of heterogeneous canopy. In this work, the RGM model at pixel scale (RGMPS model) is proposed with the open accelerator (OpenACC) acceleration techniques and two improved algorithms, which solve the overloaded view factor calculation and enhance scene availability without sacrificing accuracy. Two heterogeneous canopy scenario experiments were used for validation, including a realistic single-tree experiment and a large-scale synthetic heterogeneous canopy experiment. The RGMPS model has increased by nearly 70 times the speed of the original RGM model, demonstrating its capability to model large-scale scenes spanning ten thousand square meters. The ${R} ^{2}$ between RGM and RGMPS is over 0.94 and the root-mean-square error (RMSE) is below 0.0038. The cross-model validation between the RGMPS model and the discrete anisotropic RT (DART) model achieved a high agreement with ${R} ^{2}$ as high as 0.98 in the near-infrared (NIR) band. An assessment conducted using airborne multiangle measurements also demonstrated that the accuracy of the proposed solution was deemed satisfactory for bi-directional reflectance factor (BRF) simulation, with RMSEs of 0.0031 and 0.0340 for the red and NIR bands, respectively. Our research contributes to the development of more efficient and accurate BRF simulations for large heterogeneous canopy scenes using the RGM model.