Falling is a serious and costly health problem for senior people. It is reported that one out of three senior people around the world falls every year. As a result, a large number of fall detection and fall prevention systems are developed. In existing technologies, camera-based and inertial measurement unit-based are two dominant methods for fall prevention. Considerable radar-based researches were carried out as well, however, most of them were focused on fall detection. Therefore, this paper presents a theoretical study on a novel all-time fall prevention system using on-shoe K-band frequency-modulaed continuous-wave(FMCW) radar, which is capable of detecting the absolute distance between the radar and the obstacles in front of the shoe. The concept of fall prevention is illustrated, and a FMCW radar prototype is tested to demonstrate the feasibility of radio frequency range detector for fall prevention.