Taking the super large diameter ventilation shaft project of a highway tunnel as the research object, in view of the great risk of blockage and serious effect on the construction period of the small-diameter shaft mucking in the ventilation shaft construction center, considering a discontinuous deformation dynamic process of the shaft rockfall, a targeted probability analysis of small diameter wellbore rockfall blockage has been carried out by adopting the discontinuous deformation analysis (DDA) method based on a numerical modeling of wellbore rockfall, taking sphere as the basic block unit, with ball particles bound into blocks of corresponding shape by using the binding algorithm, followed by a simulation of the collision and friction during block movement. The numerical simulation results show that in order to avoid the blockage of the small diameter mucking shaft, the size of the rocks and the size of the blocks thrown into the mucking shaft must be strictly controlled; the more serious the over excavation and under excavation of the small diameter mucking shaft constructed by drilling and blasting method, the greater the blockage probability will be. The simulation results truly reflect the engineering practice of the working condition of the mucking shaft well.