Step-by-step direct-acting solenoid valve is a combination of direct-acting and pilot-operated valve. In normally closed condition, when there is no pressure difference between inlet and outlet, after the solenoid valve is energized, the electromagnetic force directly opens the pilot hole and connects the main valve piston, which moves upward and opens the valve; when there is a start-up pressure difference between inlet and outlet, after the solenoid valve is energized, the electromagnetic force firstly opens the pilot hole, and the pressure of the upper chamber of the main valve piston decreases, which uses the pressure difference and the electromagnetic force to pull the main piston and make the valve mouth open. In the event of power failure, the spring action resets the pilot hole to close, the main piston upper chamber pressurization, pushing the main piston down, closing the valve. In normally open type, the working principle is opposite to normally closed type.