To address the need for improving the flexibility of flexible needles, a technique that achieves needle steering by altering the needle’s stiffness is proposed. Needle tip forces were simplified, and a deflection model was then developed based on the Timoshenko beam theory. By combining theory and simulation, the influences of the stylet diameter, extension length, and tip direction on needle bending were analyzed. The deflection of the flexible needle is proportional to the cube of the stylet extension length. The needle with a 0.4 mm stylet diameter and an 8 mm extension length produces 8.841 mm more deflection than conventional flexible needles. The extension length can effectively regulate the bending curvature of the flexible needle, thus improving the flexibility of the puncture to some extent.