AbstractThe numerical manifold method (NMM) provides a general framework for simulating both continuities and discontinuities. By cutting covers, NMM can adopt a regular-shaped triangle (or quadrilateral) for interpolation. This brings advantages of the best-shaped mesh; however, it also leads to some issues in displacement boundaries. This study shows possible issues and related treatments on boundaries inaccuracies in NMM: (1) constrained partial edges, (2) cumulative errors, and (3) boundary changes in contact iteration. In NMM, constraining a part of the edge will constrain the whole edge, and therefore bring additional stiffness. This error can be avoided by a new load-spring control method or adjusting the NMM mesh. In dynamic NMM analyses, penalty springs reapply at every time step, and they only constrain incremental displacement. As a result, cumulative errors should be avoided. If time step interval changes in contact iteration, moving boundaries need to be renewed. This paper also provides general formulas for displacement boundaries, the necessary and sufficient condition for line constraints, a load-spring control for full load–deflection responses, and a robust scheme for moving boundaries.