Abstract: For the underground tunnel, it is difficult to evaluate its stability using the classical analysis method applicable to soil slopes, since its failure surface and failure mode are very different from those of the slopes. In this paper, incorporated with the concept of strength reduction, the approach of calculating acceleration is extended to evaluation of the safety factor of underground tunnels, which is defined as the reduction factor that minimizes the magnitude of acceleration. We generate the failure surface using a smooth continuous curve expressed in polar coordinates, and adopt ant colony optimization for continuous domains to search the minimum safety factor and critical failure surface. Thus, what we are developing is actually a new explicit method for calculating the acceleration-based safety factor of underground tunnels. It has the same theoretical basis as those for calculating the safety factor of slopes, and is shown to be effective through a case study of the stability analysis of an underground tunnel.