The invention provides an oil injection device for a hydraulic unit of a rotary guide tool. The oil injection device comprises an oil storage unit, a vacuum negative-pressure unit, a liquid flow driving unit, a clamp unit and an adjusting unit, wherein the oil storage unit communicates with the hydraulic unit through a circulating loop, the vacuum negative-pressure unit is connected with the oil storage unit, the liquid flow driving unit is arranged on the circulating loop, the clamp unit clamps the hydraulic unit, and the adjusting unit is connected with the clamp unit and can adjust the placement posture of the clamp unit. Through configuration support, vacuumizing, circulating and synchronous heating and oil injection operation is achieved, meanwhile, the special oil filling clamp is arranged so as to adjust the placing direction of the hydraulic unit of the rotary guide tool in the oil filling process, thus residual air in an oil filling cavity in the hydraulic unit is reduced, andthe work efficiency of oil injection is improved.