Stitching interferometry is an effective method to extend the measurement range of commercial interferometer. It has been applied in the laboratorial environment, but rarely in workshop. In order to improve the testing efficiency in workshop, stitching interferometry could be combined with machine tool and implement in-situ testing. A dynamic stitching interferometer system is established in this paper, which contains dynamic interferometry, precision motion control and advanced stitching algorithm. This system has been prepared for the in-situ testing of large plano optics. One example optical flat with size 200mm×300mm was used to verify the feasibility and accuracy of this system. Many repetitive experiments have been proved the well reliability of the system and method.