The invention discloses a system and method for supplying power to on-board electrical appliances when a fuel vehicle is parked. The system includes: a vehicle generator, a vehicle lead-acid battery and a lithium battery system installed in the fuel vehicle; the vehicle lead-acid battery and a lithium battery system The battery system is connected in parallel with the vehicle generator to form a charging circuit; the vehicle lead-acid battery is connected with each vehicle electrical appliance to form a power supply circuit; the lithium battery system is connected with each parking vehicle electrical appliance to form a discharge circuit; the lithium battery system includes a lithium ion battery module. Group, high-voltage components, box, battery management system protection board with charging current limiting circuit and power supply voltage detection circuit, temperature sensor and heating component; lithium-ion battery module, high-voltage components, battery management system protection board are connected in sequence The charging current limiting circuit is arranged in series between the vehicle generator and the lithium battery system, and the power supply voltage detection circuit is arranged in series in the discharge circuit. The invention has the advantages of low use cost, safety, long service life, environmental protection and long-term effective and stable operation.