A solenoid valve and an assembly method thereof are provided. The solenoid valve includes a valve body assembly. The valve body assembly is provided with a valve cavity and a main valve port. The solenoid valve further includes a first iron core, a second iron core, a valve core assembly and a piston assembly disposed in the valve cavity. The valve core assembly includes a sealing portion and a valve rod portion. The valve core assembly is movably in sealing fit with the guiding valve port via the sealing portion. One end of the valve rod portion is movably inserted into the first cavity and is configured to be stopped in the first cavity, another end of the valve rod portion passes through the second cavity and is connected to the sealing portion, and the sealing portion is configured to be stopped outside the second cavity.