The present invention provides a system and method for economically using compressed air as automobile power source, comprising: a compressed air power device, which includes automobile air storage tubes (1) to store a sufficient amount of high-pressure compressed air and a cylinder-combined engine consisting of the first and second cylinders (9)(10), and which can make full use of the compressed air to produce driving power; a mechanism to produce, store and provide high-pressure compressed air, which includes a boiler-type high-pressure compressed air producing and storing device, abbreviated as boiler-type HCAPS device (4), to be able to use electricity during periods of low energy demand (off-peak) such as at night simultaneously recovering the by-produced heat for central heating, and pressurizing and inflating into the automobile air storage tubes (1) during daytimes; brake energy recovery and regeneration devices, which include a spring reserving-releasing device and/or a compressed air reserving-releasing device to save the compressed air in the automobile air storage tubes (1) for saving the driving power; an inner gear ring assembly, which includes an inner gear ring (2) gearing meshing with inner acting gears (45), with the first and second accelerating gears (72)(92), with a flywheel front inner meshing gear (48) and reset gears (46), for transmitting torque and mixing/outputting power; some clutch transmission devices and a controller, which controls orderly coordinated operation of devices and mechanisms.