The conventional cross-section size design of the utility tunnel relies too much on the subjective judgment of the relevant normative values and experience, resulting in a series of problems such as insufficient utilization of space. As a result, a method for optimizing the cross-section size design of the utility tunnel based on nonlinear programming is proposed to improve the utilization rate of utility tunnel space and reduce the construction investment cost. The mathematical model is established and the case optimization is calculated by software Matlab to make the design result more scientific and reasonable. The feasibility of the method is verified according to case analysis. The results show that under the condition of controlling cost, the cross-section area is increased by 20.8%, which meets the requirements of the relative codes and improves the space utilization rate as well.