In this paper, numerical simulation and orthogonal experiment are carried out to study the safe distance between karst tunnels and karst caves. To perfect the model to predict the safe distance between karst tunnels and karst caves underneath, the authors develop a 3D model, which predicts the safe distance based on the following five factors: the depth of the tunnel, the level of the surrounding rock, the span of the karst cave, the base angle, and the ratio between the height and the span of the cave. The 3D model has been trialed in actual projects and the prediction results turn out to be very close to the space distance that was adopted, therefore proving the reliability of the model.