Abstract: The rational design of a supporting structure is key to the design of loess tunnels with large sections. Based on a specific loess tunnel, this paper analyzes the support structure, force upon the surrounding rock, and mechanical behavior variation at different stages of support of the loess tunnel. A rational suggestion for timing the selection of primary support is given with a focus on controlled rock deformation. This paper summarizes the law of force upon the primary support of loess tunnels and comes to the following conclusions: the loess features rheological properties and the additional load induced by the rock due to this is mainly born by the support structure; and the force acting upon the support structure is asymmetrical and should be taken into consideration in the design.