A power supply guarantee system, applied to a battery management system, characterized in that the power supply guarantee system comprises: a main control module, a high voltage battery pack, a high voltage power module, a timing device, and a power conversion module, wherein the main control module is configured to send a received wake-up time to the timing device; the high voltage power module is configured to power the timing device according to electrical energy in the high voltage battery pack; the timing device is configured to set a wake-up clock according to the wake-up time, start timing when the battery management system enters sleep, and send a discharge instruction to the high voltage battery pack when the timing reaches the wake-up time; and the power conversion module is configured to convert high voltage electrical energy into low voltage electrical energy, and use the low voltage electrical energy to power the battery management system, wherein the high voltage electrical energy is output by the high voltage battery pack according to the discharge instruction, wherein the power conversion module comprises: a forward power control module, a first high voltage transmission module, a synchronous rectification module, a first isolation and driving module, and a second high voltage transmission module, wherein the first high voltage transmission module and the second high voltage transmission module are not the same module, wherein the timing device is further configured to provide an enable signal to the forward power control module when the timing reaches the wake-up time, so that the forward power control module starts working under the control of the enable signal; the forward power control module is configured to provide a power supply to the synchronous rectification module through the first high voltage transmission module; and the second high voltage transmission module is configured to convert high voltage electrical energy in the high voltage battery pack into low voltage electrical energy when the connection between the second high voltage transmission module and the high voltage battery pack is turned on; and the synchronous rectification module is configured to perform synchronous rectification on the low voltage electrical energy, and use the synchronously rectified low voltage electrical energy to power the battery management system, characterized in that the forward power control module is configured to control an on-state of connection between the high voltage battery pack and the second high voltage transmission module through the first isolation and driving module. the main control module is configured to send a received wake-up time to the timing device; the high voltage power module is configured to power the timing device according to electrical energy in the high voltage battery pack; the timing device is configured to set a wake-up clock according to the wake-up time, start timing when the battery management system enters sleep, and send a discharge instruction to the high voltage battery pack when the timing reaches the wake-up time; and the power conversion module is configured to convert high voltage electrical energy into low voltage electrical energy, and use the low voltage electrical energy to power the battery management system, wherein the high voltage electrical energy is output by the high voltage battery pack according to the discharge instruction, wherein the power conversion module comprises: a forward power control module, a first high voltage transmission module, a synchronous rectification module, a first isolation and driving module, and a second high voltage transmission module, wherein the first high voltage transmission module and the second high voltage transmission module are not the same module, wherein the timing device is further configured to provide an enable signal to the forward power control module when the timing reaches the wake-up time, so that the forward power control module starts working under the control of the enable signal; the forward power control module is configured to provide a power supply to the synchronous rectification module through the first high voltage transmission module; and the second high voltage transmission module is configured to convert high voltage electrical energy in the high voltage battery pack into low voltage electrical energy when the connection between the second high voltage transmission module and the high voltage battery pack is turned on; and the synchronous rectification module is configured to perform synchronous rectification on the low voltage electrical energy, and use the synchronously rectified low voltage electrical energy to power the battery management system, characterized in that the forward power control module is configured to control an on-state of connection between the high voltage battery pack and the second high voltage transmission module through the first isolation and driving module.