To address the transition problem between the W-beam guardrail and the combined bridge guardrail during the upgrading and reconstruction of in-service highway guardrails, an SB-level wing-wall-free transition section with simple construction and high efficiency was proposed. Computer simulation technology was adopted to study the stiffness transition, height transition, and cross-section transition of the wing-wall-free transition section. The guardrail stiffness transition was achieved by optimizing the column spacing and column cross-sectional dimensions. By comparing the contact area between the vehicle and the guardrail and the vehicle redirection situation under different height differences, it was preliminarily determined that the vehicle can be safely redirected when the height difference of the guardrail is less than 15 cm. On this basis, a collision simulation analysis was conducted on the specific scheme of the wing-wall-free transition section, and its protection performance was further verified through a real vehicle collision test. The test results indicate that the structure can achieve the SB-level protection capacity stipulated by the standard.