An energy-saving oil drilling rig is researched. A large accumulator is adopted in this rig to store the energy of the motor during the auxiliary time of lifting the drill stem and the potential energy of the drill stem when lowered. The equipped power of this rig decreases remarkably compared with the conventional drilling rig, and this rig can also recover and reuse the potential energy of the drill stem. Therefore, this rig owns remarkable energy-saving effect compared with the conventional drilling rig, and the energy-saving effect of the energy-saving oil drilling rig is also verified by the field tests. The mathematical model of the energy-saving oil drilling rig lowering the drill stem was derived and simulation analysis was conducted. Through simulation the curves of the drill stem lowering velocity and lowering displacement with time were obtained, and some conclusions were reached: (1) the heavier the drill stem lowered, the higher the lowering velocity is, and the shorter the lowering time is; (2) the smaller the displacement of the variable pump-motor, the higher the lowering velocity is, and the shorter the lowering time is. (C) 2010 Elsevier Ltd. All rights reserved.