Abstract: As an important part of main insulation of power transformer, insulating oil is also the information feedback carrier of equipment running state and health state. It mainly plays the role of insulation, heat dissipation and ARC extinging in transformer, the insulation performance of insulating oil, that is, the breakdown voltage, directly reflects the main insulation condition of the equipment, which is an important basis for the tester to judge the operation condition of the main transformer. Air Bubble, water and solid impurity particles in insulating oil are the main factors that affect the breakdown voltage of insulating oil. The Test Oil Cup used in the insulation oil breakdown voltage meter is an open structure, and it is easy to be polluted by the test personnel and the ode environment during the operation, which leads to the data distortion, thus misjudging the running state of the equipment, in order to solve the problems such as the loss of representativeness of insulating oil samples caused by external factors, the lack of accuracy and repeatability of testing data, a set of insulating oil sampling assistant system and a fully sealed oil cup were developed in this paper.