For tokamak devices, neutral beam injection (NBI) remains an important method for heating and current drive. In this paper, the beam transmission model based on positive ion source neutral beam injector (P-NBI) is constructed using the Monte Carlo method to design beamline system for burning plasma tokamak, which needs NBI with energy of 120 keV and power of 15 MW. The neutralization, deflection, and transmission of beam particles are investigated, and the effects of beam quality, gas supply rate, and structural parameters are analyzed in detail, including transmission efficiency, reionization loss and the deposited power density of the beamline components at different modes of NBI system. The results show that the neutralization efficiency can reach 41.4%, the transmission efficiency exceeds 80.7%, the reionization loss is calculated to be <10%, and the system is capable of injecting 15 MW of deuterium neutral beam into the plasma when beam divergence angle is ∼1°. This work presents the complete beamline design process and will provide guidance for developing NBI systems. As a key component of a burning plasma tokamak, it will strongly influence the experimental results. Currently, the components are being fabricated and experiments will commence shortly.