The utility model discloses a small-size oil pipe hydraulic cutting knife which comprises an upper connector, a lower connector, a thrust rod, a reset spring and a cutting knife body, the thrust rod is installed in the lower connector, the tail end of the thrust rod forms a piston structure in the lower connector, and the reset spring provides restoring force for the thrust rod. A limiting pin isarranged on the lower connector, a guide groove is formed in the thrust rod, and the bottom end of the limiting pin is matched with the groove to achieve limiting of the thrust rod. The number of thecutting knife bodies is two. The two cutting knife bodies are symmetrically installed. Cutter teeth are embedded in the outer ends of the cutter bodies, the inner ends of the cutter bodies are hingedto the lower connector through fixing pins, the opposite faces of the two cutter bodies form a section of conical structure in a closed state, and when the thrust rod is driven to move outwards, the outer end of the thrust rod pushes the opposite conical faces of the two cutter bodies, so that the two cutter bodies are opened. The utility model has the characteristics that the cutting knife is simple to fold and unfold, safe and reliable, strong in universality and wide in adaptability.