The invention relates to an insulation test method of a 750kV converter transformer three-core column outgoing line device. The method comprises the following steps: firstly, establishing a system forperforming an insulation test of the 750kV converter transformer three-core column outgoing line device, inserting an inner lead into a roasted aluminum shielding tube in the three-core column outgoing line device to fix the same on a lower end of the aluminum shielding tube, twining and fixing a voltage sharing ball on the lower end of the aluminum shielding tube of the three-core column outgoing line device through crepe paper by a fixing conductor grip, and connecting the voltage sharing ball with the aluminum shielding tube by using an equipotential line to achieve an equipotential; then,mounting the three-core column outgoing line device on a lower ascending flanged base of an insulation test system, starting a first valve to connect a pipeline, vacuumizing to 30 Pa, and holding for12 hours; and performing hot oil circulation for 6 hours according to an upper-inlet-lower-outlet manner, stewing for 72 hours, and performing the insulation test of the three-core column outgoing line device. The insulation test method provided by the invention has the functions of vacuum oil injection and hot oil circulation, meets the requirements of insulation tests, such as partial discharge, and avoids the damage to the internal insulation of a transformer when being installed in the transformer for test.