The invention discloses a device and a method for detecting the remaining power of a battery. The device comprises a single chip microcomputer, a current and voltage regulating module, a voltage sampling circuit, a current sampling circuit, and a PWM controller. The current sampling circuit is connected in series between the current and voltage regulating module and a battery. The voltage sampling circuit and the battery are connected in parallel into the current and voltage regulating module. The current and voltage regulating module comprises a light emitting diode and a photosensitive resistor. The PWM controller is used for controlling the brightness of the light emitting diode to control the value of photosensitive resistance so as to obtain voltage and current sampling values, and the obtained voltage and current sampling values are sent to the single chip microcomputer for processing to obtain the remaining power of the battery. The single chip microcomputer is connected with a display and a power alarm device. By using the method, the voltage across the two ends of a battery can be measured accurately in the starting, running and decelerating phases of an electric vehicle, the voltage across the two ends of the battery can be obtained accurately even if the battery is just fully charged or the remaining power of the battery is low, and the remaining power of the battery can be calculated accurately.