The invention discloses a hydraulic oil cylinder and an assembly method. The hydraulic oil cylinder comprises a base, guide rods, a head cover, a cylinder base, a cylinder barrel, a cylinder cover, atransmission component, a piston rod, a sealing and dustproof component, a piston, an oil inlet pipeline and an oil outlet pipeline, wherein the periphery of the top of the base is connected to the head cover by the guide rods; the cylinder base is arranged on the top of the base; the cylinder barrel is arranged on the top of the cylinder base; the cylinder cover is arranged on the top of the cylinder barrel; the sealing and dustproof component is arranged inside the cylinder barrel; the transmission component is arranged at the bottom of the head cover; the sealing and dustproof component comprises an annular fixture block, an annular barrel plug, a first sealing ring, an annular clamping groove, a second sealing ring, an upper dustproof ring and a lower dustproof ring; the annular clamping groove is formed in the top of the inner wall of the cylinder barrel; and the outer side of the annular barrel plug is in sleeve connection with the annular fixture block. The hydraulic oil cylinder has the beneficial effects that the first sealing ring and the second sealing ring facilitate sealing, the upper dustproof ring and the lower dustproof ring facilitate dust prevention, the hydraulicworking is performed through mechanical transmission, and the transmission is stable.