The invention discloses a built-in guide oil cylinder which comprises a lower end cover, a cylinder barrel, a right guide rod, a left guide rod, an upper end cover, a plunger piston and the like. A guide device is mounted inside an oil cylinder, so that in a movement process of the oil cylinder, on one hand, radial guide effect of the oil cylinder is reinforced, on the other hand, rotation of the oil cylinder is restricted, and therefore, circumferential-direction guide of the oil cylinder is reliably realized. The built-in guide oil cylinder is simple in structure, reliable in work, solves the problem that a guide device for external accessories of the oil cylinder is limited by a mounting space and conditions, can be applied to limiting a circumferential-direction rotation occasion in a movement process of the oil cylinder, and has a wide application prospect.