Abstract: Complex geological conditions and information gaps often necessitate design alterations during tunnel construction. The alteration of rock mass classifications makes it especially difficult to control the construction schedule and cost. Considering the effects of the alteration of surrounding rock classifications on the construction period and cost, a Monte Carlo Simulation-based model was established by using the alteration probability of each rock mass classification, construction time, and cost per linear meter as random variables. Through the empirical study of a double-track tunnel, the practicability and feasibility of the model was verified, and a new method is provided for reasonably determining and assessing the construction schedule and cost of a tunnel.