Bronchoscopy, as an essential minimally invasive diagnostic and therapeutic modality, assumes a pivotal role in the early detection of lung cancer. However, the complex anatomy of the airway and the fixed length of the bronchoscope’s bending segment, along with its external propulsion property, pose challenges, including the risk of bleeding. This paper introduces a 4 mm diameter robot-assisted bronchoscope with a spring-based extensible segment. By manipulating two driven rods, the segment can be lengthened or shortened. The advantages of the extensible segment are discussed in two main aspects through theoretical analysis and experimentation. Firstly, the extensible segment enables the bronchoscope to move in a follow-the-leader motion mode or fixed-angle motion mode, navigating through narrow corners that are inaccessible to fixed-length bronchoscopes. It can also be shortened to increase its stiffness when it reaches the target position, creating a stable surgical platform for procedures like biopsies. In addition, a tailored master device has been developed to control the extensible bronchoscope in an isotropic manner. Phantom experiments confirm the feasibility and effectiveness of the extensible bronchoscope.