The prototype for a negative hydrogen ion source for neutral beam injection of China Fusion Engineering Test Reactor is being developed at the Southwest Institute of Physics. To study the physics of negative ion beam transport and to optimize the design of the source, the stripping loss and the stray particles' impacts on the one-RF-driver prototype are analyzed. Collision simulation, including both the beam-gas collisions and the particle-grid collisions, is carried out basing on the results of gas flow evaluation and particle tracing. The stripping loss, the distribution of stray particles and the heat loads are calculated, comparing two configurations of grounded grid (GG) (multiaperture or multislot). At the source filling pressure of 0.3 Pa and the vessel pressure 0.05 Pa, the extraction voltage being 8 kV and the acceleration voltage 200 kV and the extraction grid (EG) magnet peak of ±45 mT, the stripping loss of the 200 A m−2 H− beam can be reduced from 25% to 20% by changing GG from multiapertures to multislots. The H− proportion in the total current at 40 mm after GG, however, shows smaller change than the reduction of the stripping loss possibly because the multislot GG's larger transparency increases the chance for the stray particles to pass through GG. The total heat load on EG in the two cases with different GG configurations are both around 66 kW, while the GG heat load is reduced from 45 kW for multiapertures to 17 kW for multislots. The study provides good comprehension of the transport process and useful guidance for practical operations.