Critical size lunar soil particles refer to particles with a particle size similar to the diameter of the drill bit, which is an important factor affecting lunar soil drilling sampling, and its influence is uncertain. In this paper, the discrete element method is used to simulate the drilling sampling process of the critical size lunar soil particles and the influence of the size, position and shape of the lunar particles with critical diameter on the drilling load is analysed. It is found that the critical size lunar soil particles have special migration trajectories and load characteristics during drilling sampling. The size and shape of lunar soil particles are the key factors affecting the drilling load. This study can support the design of drilling tools for lunar soil exploration and sampling.