Discontinuous deformation analysis (DDA) is a dynamic calculation method for solving problem in an implicit pattern. It adds or subtracts stiff springs between block interfaces to satisfy the contact conditions of neither tension nor embedment. It is found that both of time step and spring stiffness directly impact the calculation results in the process of DDA. This paper investigates the role of inertial force during solving process of DDA based on adjustment of time step and spring stiffness during DDA and simplified mechanical conceptual model for block contact problems. Numerical simulation tests are conducted to study some concerned mechanical problems under three mechanical states in free falling model and single block sliding model on incline respectively. By simulating the free falling movement, this paper studies the influence of value selection of time step on calculation result and determines a proper range of time step preliminarily. Further, single bock sliding model is used to analyze the joint effect of time step and spring stiffness. A proper range for spring stiffness corresponding to different time steps is obtained. The results show that the proper ranges of time step and spring stiffness constitute a simply-connected domain. When the values of time step and spring stiffness are both within the proposed domain, the result of DDA is rational.