The invention relates to a method for recycling base oil of the waste liquid of an oil-based drilling liquid, and in particular belongs to a solid-liquid separation technique in the technical field of waste oil-based drilling liquid treatment. The method comprises the following steps of: (1) preparing a composite demulsifying agent, namely compounding AE, BP, AP and OP-10 or NP-10, wherein the mass ratio of the AE to the BP to the AP to the OP-10 or the NP-10 is 1 to 2 to 2 to 1, 2 to 1to 1 to 2, 1to 3to 3 to 2 or 2 to 3 to 2 to 1; (2) demulsifying and flocculating, namely under a stirring state, adding a polymeric flocculant, the composite demulsifying agent, an anionic surfactant and a cationic surfactant into the waste liquid of the oil-based drilling liquid; and (3) carrying out solid-liquid separation. The method provided by the invention is suitable for recycling and treatment of the base oil of the waste liquid of various oil-based drilling liquids, the technology is simple, no heating is needed, the investment is less, the cost is low, and the recycling rate of the oil is high; the recycled base oil is used for preparing the drilling liquid again, the performance of the drilling liquid is not changed, so that harmlessness and reclamation of the drilling liquid are realized, and energy is saved.