To solve the problem of the low precision of collision detection in the turn-mill combined machining simulation, a rapid collision detection algorithm based on the improved theory of Oriented Bounding Box was proposed. This algorithm, based on the improved oriented bounding box, and using the collision detection result as the input, creates the oriented bounding box and hierarchical bounding box tree of the collision unit group. During the rough detection phase, the intersecting nodes are detected. In the following precise detection phase, the triangular patches of the objects in collision are detected. With the application of feedback technology, the initial time and position of the collision are calculated. A case study shows that this algorithm increases the efficiency and precision of the collision detection in numerical control simulation.