The invention discloses an on-line optimization method for energy consumption of oil field mechanical oil extraction system. The method comprises: (1) determining an oil well IPR curve; (2) performingmulti-phase pipe flow calculation, specifically, performing multi-phase pipe flow calculation upwards through flow pressure, pressure distribution at different depths is obtained, and determining thepump descending depth according to specified pump inlet pressure; (3) optimally designing a sucker rod string, a stroke, a stroke frequency and the like, combining certain swabbing parameters, namelya pump diameter, a stroke frequency and a stroke frequency, calculating maximum and minimum loads of the sucker rod string, and checking and designing the sucker rod string; (4) checking equipment such as an oil pumping unit and a motor; (5) calculating an energy efficiency index. The method has the advantages that an energy efficiency optimization theory calculation model is innovatively established to simulate and calculate the power consumption of the rod-pumped well. The energy efficiency optimization design of the rod-pumped well is carried out, the system efficiency of the rod-pumped well can be effectively improved, the production energy consumption of the rod-pumped well is remarkably reduced, the pump inspection period is prolonged, and therefore low-cost development is achieved.