The invention belongs to the technical field of under-pressure cable operation in oil-gas fields and gas wells, and particularly provides an under-pressure cable injector. The under-pressure cable injector comprises a shell body, an upper joint, a lower joint, a hydraulic oil injection joint, a pushing mechanism, a hydraulic oil return joint, a power wheel unit, a pushing support and a power wheel support. The upper joint and the lower joint are located on the upper portion and the lower portion of the shell body correspondingly. The pushing mechanism, the power wheel unit, the pushing support and the power wheel support are arranged in the shell body, the pushing support is located in the pushing mechanism, and the power wheel unit is located on the power wheel support. An inlet of an upper inner chamber of the pushing mechanism communicates with the hydraulic oil injection joint. One end of the upper portion of the pushing support communicates with an outlet of the upper inner chamber, and the other end of the upper portion of the pushing support communicates with the power wheel unit. One end of the lower portion of the upper inner chamber communicates with the power wheel support, and the other end of the lower portion of the upper inner chamber communicates with an inlet of a lower inner chamber. An outlet of the lower inner chamber communicates with the hydraulic oil return joint. The under-pressure cable injector solves problems of various insecurities caused by normal counterweights, and achieves the effect that cables, operation instruments and tool strings are placed downwards in a well with little weight-adding and even without weight-adding.