The negative ion based neutral beam injection (NNBI) system is one of the testing or demonstrating systems in the frameworks of Comprehensive Research Facility of Fusion Technology (CRAFT). The object of the CRAFT NNBI system is to research the key physics and engineering issues around the NNBI, and to accumulate experience for future development and operation of the NNBI system for fusion reactor. The beamlet optics character of a negative ion accelerator determines the divergence of the formed beam, and further influences the beam transmission efficiency through the accelerator and the beamline, which is very important to the high-power, high-energy, and long-pulse operation of the NNBI system. Therefore, the ion beam simulation code IBSimu was used to analyze and estimate the physics design of the beamlet optics of the 400 keV accelerator for the CRAFT NNBI system. The IBSimu code has been successfully benchmarked and applied to many negative ion sources. The current design of the electrode aperture has a similar structure of the ITER negative ion source, the calculation results of the beamlet divergence can meet the design requirement. A higher extracted ion current density (between 100 to 300 A/m²) draws a lower beamlet divergence. When properly increasing the extraction gap (between 5 to 7 mm) or acceleration gap (between 88 to 110 mm), there is a decreasing tendency of the beamlet divergence.