The invention discloses a multistage step-up system for an X-ray machine. The multistage step-up system comprises a lithium battery pack, a liquid crystal display circuit, a master control unit, a high-frequency step-up circuit, an X-ray tube and a voltage-multiplying rectifying circuit. An inverter step-up circuit and a pulse width modulation circuit are further connected in series between the lithium battery pack and the high-frequency step-up circuit. The lithium battery pack and the inverter step-up circuit constitute a primary step-up circuit. The pulse width modulation circuit, the highfrequency step-up circuit, the voltage-multiplying rectifying circuit, and the X-ray tube constitute a secondary high-voltage step-up circuit. The liquid crystal display circuit and the master controlunit constitute a display/control circuit to display and control the exposure time. The master control unit, the pulse width modulation circuit, and the X-ray tube constitute a filament drive circuitthat controls the magnitude of the current of an exposure tube. The multistage step-up system is simple in structure, reasonable in design, safe and reliable, can increase the 24V voltage of the lithium battery pack to 70kV, and can be directly applied to a medical portable X-ray dental machine.