This work considers the control of underactuated bipedal walking, and a novel capturability-based control framework is presented. Compared with traditional approaches, the presented control method does not rely on the use of the Poincaré map, which may take significant computational cost. Firstly, a new definition of stable walking is presented, and a novel foot-placement based control method is proposed. Then, a controller design method is presented based on this control method. For the controller design, the foot placement adjustment is achieved by updating the virtual constraints using a heuristic method, and an improved virtual constraint control method is proposed to enforce the virtual constraints. Finally, the effectiveness of the presented control framework is illustrated on a five-link underactuated planar biped by numerical simulations.