It is difficult to accurately calculate the lump coal rate in a fully mechanized mining face. Therefore, a numerical simulation of the coal wall cutting process, which revealed the crack expansion, development, evolution in the coal body and the corresponding lump coal formation mechanism, was performed in PFC. Moreover, a correlation was established between the cutting force and lump coal formation, and a statistical analysis method was proposed to determine the lump coal rate. The following conclusions were drawn from the results. (1) Based on a soft ball model, a coal wall cutting model was established. By setting the roller parameters based on linear bonding and simulating the roller cutting process of the coal body, the coal wall cutting process was effectively simulated, and accurate lump coal rate statistics were provided. (2) Under the cutting stress, the coal body in the working face underwent three stages—microfracture generation, fracture expansion, and fracture penetration—to form lump coal, in which the fracture direction was orthogonal to the cutting pressure chain. Within a certain range from the roller, as the cutting depth of the roller increased, the number of new fractures in the coal body first increased and then stabilized. (3) Under the cutting stress, the fractured coal body was locally compressed, thereby forming a compact core. The formation and destruction of the compact core caused fluctuations in the cutting force. The fluctuation amplitude was positively related to the coal mass. (4) Because the simulation did not consider secondary damage in the coal, the simulated lump coal rate was larger than the actual lump coal rate in the working face; this deviation was mainly concentrated in large lump coal with a diameter greater than 300 mm.