A heavy oil hydrotreating system, comprising a prehydrotreating reaction zone, a transition reaction zone, and a hydrotreating reaction zone that are connected in series, and sensor units and a control unit, wherein the sensor units are arranged to detect pressure drop in each prehydrotreating reactor in the prehydrotreating reaction zone, and the control unit is arranged to receive pressure drop signals from the sensor units; in the initial reaction stage, the prehydrotreating reaction zone includes 3-6 prehydrotreating reactors (A, B, C) connected in parallel, and, in the initial reaction stage, the transition reaction zone doesn't include any prehydrotreating reactor; in the reaction process, the control unit is arranged to control material feeding to and material discharging from the prehydrotreating reactors in the prehydrotreating reaction zone according to pressure drop signals from the sensor units, so that: when the pressure drop in one prehydrotreating reactor reaches the predetermined value, the prehydrotreating reactor is switched from the prehydrotreating reaction zone to the transition reaction zone, and is named as a cut-out prehydrotreating reactor I, and the prehydrotreating reaction zone, the cut-out prehydrotreating reactor I, and the hydrotreating reaction zone are connected in series successively; when the pressure drop in the next one prehydrotreating reactor reaches the predetermined value, the prehydrotreating reactor is switched from the prehydrotreating reaction zone to the transition reaction zone, and is named as a cut-out prehydrotreating reactor II, and the prehydrotreating reaction zone, the cut-out prehydrotreating reactor II, the cut-out prehydrotreating reactor I, and the hydrotreating reaction zone are connected in series successively; the other prehydrotreating reactors are treated in the above-mentioned method, till all of the prehydrotreating reactors are connected in series; preferably, the predetermined value of pressure drop in the prehydrotreating reactor is 50%-80% of a design upper limit of pressure drop for the prehydrotreating reactors, more preferably is 60%-70% of the design upper limit of pressure drop. when the pressure drop in one prehydrotreating reactor reaches the predetermined value, the prehydrotreating reactor is switched from the prehydrotreating reaction zone to the transition reaction zone, and is named as a cut-out prehydrotreating reactor I, and the prehydrotreating reaction zone, the cut-out prehydrotreating reactor I, and the hydrotreating reaction zone are connected in series successively; when the pressure drop in the next one prehydrotreating reactor reaches the predetermined value, the prehydrotreating reactor is switched from the prehydrotreating reaction zone to the transition reaction zone, and is named as a cut-out prehydrotreating reactor II, and the prehydrotreating reaction zone, the cut-out prehydrotreating reactor II, the cut-out prehydrotreating reactor I, and the hydrotreating reaction zone are connected in series successively; the other prehydrotreating reactors are treated in the above-mentioned method, till all of the prehydrotreating reactors are connected in series; preferably, the predetermined value of pressure drop in the prehydrotreating reactor is 50%-80% of a design upper limit of pressure drop for the prehydrotreating reactors, more preferably is 60%-70% of the design upper limit of pressure drop.