Abstract: As the basic principle used in modern underground engineering, the New Austrian Tunneling Method focuses on providing timely and optimized safe support to develop the maximum self-supporting capacity of the rock or soil itself for the stability of the underground opening. But up to now, no reliable formula and theoretical guidance for timely support have been established. The support time can only be empirically determined based on the in-situ monitoring measurements. The timely and optimized support which uses adequate self-bearing capability of surrounding rock means that the rock stress is in the final stage of elastic deformation, but before brittle failure occurs when reaching the critical elastic strain limit. Based on this idea and the characteristics of time-dependent deformation of underground engineering, an approximation formula to calculate the optimum support time is proposed. The study shows that the optimum support time is related to the deformation convergence time, ratio of rock strength to geo-stress, stress after excavation and anchorage pressure. The optimum support time for top arch and side walls can be determined according to the stresses of surrounding rock from point to point. This study may provide a theoretical method to determine the optimum support time which is a critical problem of underground engineering for a long time.