The timing of the second layer primary support in soft rock tunnels with large deformation is crucial for controlling surrounding rock deformation. Applying it too early or too late can threaten tunnel stability and economic efficiency. Therefore, determining the optimal timing for the second layer is a core issue for engineering safety and cost control. A time-varying virtual support force was introduced to describe the spatial effect of the excavation face based on viscoelastic-plastic mechanics and combines the Nishihara model to characterize the time effect of rock mass creep. A construction mechanics model for soft rock tunnels with large deformation considering spatiotemporal effects was established. The occurrence of the viscoplastic failure zone in the surrounding rock was used as the criterion for determining the optimal timing of the second layer primary support, and an analytical solution for its optimal timing was derived. Using the Xiejiapo Tunnel Project on the Anlan Expressway as an example, this model was applied to determine that the optimal timing for the second layer primary support in the soft rock section with large deformation was after the 9th excavation advance. Numerical simulation and field monitoring results show that the analytical solution is reliable and has a significant deformation control effect, effectively avoiding support structure failure. The research results provide a theoretical basis and practical method for the design of soft rock tunnel support.