The present application discloses a system and method for providing a constant power source. The system includes: a main control module configured to receive a wake-up time and send the wake-up time to a timer device; a timer power supply module configured to supply power to the timer device according to electric energy in a high-voltage battery pack; a high-voltage power supply module configured to supply power to a power conversion module according to the electric energy in the high-voltage battery pack; the timer device configured to set a wake-up clock according to the wake-up time, start timing when a battery management system (BMS) enters into sleep, and send a discharge instruction to the high-voltage battery pack when the timing reaches the wake-up time; the power conversion module configured to convert high-voltage electric energy output from the high-voltage battery pack into low-voltage electric energy and supply power to the BMS.