A velocity-detecting device is mainly based on detection of charge, including a circuit board provided thereon with at least two capacitance sensors with potential difference. The two capacitance sensors are provided at different locations for detecting the time difference when a living thing moves and has its charge respectively reaching the critical one preset by the two capacitance sensors for calculating the moving velocity of the living thing, able to detect and calculate the moving velocity of a living thing for carrying out relative reaction to a machine.