An electricity generation system includes a pneumatic generator, an air compressor, a battery module, a control module, a gas ejection module, an output circuit, a loopback circuit, and a switch. The control module controls the battery module to supply power to the air compressor, so that the air compressor is activated to generate and store compressed air in the gas cylinder. The control module controls the gas ejection module to allow the compressed air stored in the gas cylinder to enter the pneumatic generator so that the pneumatic generator generates electrical energy. The output circuit is connected to the pneumatic generator and the device to be powered. The loopback circuit connects the pneumatic generator and the battery module. The electrical energy generated by the pneumatic generator supplies the power supply device or is stored in the battery module when the switch is turned on by the control module.