Abstract: The smooth-joint (SJ) contact in Particle Flow Code (PFC) has been widely applied to mimic the mechanical behavior of discontinuity in rock mass. This contact model allows particles to have considerable overlap by moving parallel with the SJ contact plane. However, large overlap can also be introduced due to particle movement normal to the SJ contact plane in a few scenarios. The latter overlap often renders the particle band of SJ contacts failed to transmit normal stress and to mimic the mechanical behavior of discontinuity in rock mass. Such overlap is thus named as ‘abnormal overlap’. To reveal the mechanism of its appearance, this study analyzes the influence of translational motion and rotation on the normal contact force according to the principles of SJ contact model. The results suggest that particle rotation is the fundamental reason of the abnormal overlap. Specifically, particle rotation can produce positive increment (tension) of normal contact force, which offsets the negative increment (compression) of normal contact force resulting from particle overlapping normal to SJ contact plane. Then, the normal contact force could keep constant or even smaller as such overlapping continues, leading to the abnormal overlap finally. Following discussions illustrate the possibility, direction and, the corresponding influence of particle rotation on the normal contact force. Two measures (restrain rotation or modify formulation of normal contact force) are proposed finally to preclude the appearance of abnormal overlap, and one of them is validated by simulation.