The invention provides a sampling method for the particle-size test of ultrahigh-pressure insulating oil. The sampling method is characterized by adopting the following steps: (1) a latex pipe with a glass pipe at the tail end is connected to a sampling valve, the sampling valve is opened for washing a sampling pipeline; (2) a special sampling bottle for the particle size test is arranged on a working platform surface in a clean environment, a sealing cover of the sampling bottle is removed, a clean cover with a hole at the top part seals and covers the sampling bottle, and the glass pipe is inserted to the bottom part of the sampling bottle from the hole in a sealing manner; (3) the sampling valve is opened, an oil sample flows into the sampling bottle to the position 0.5-1cm away from the sealing cover of the sampling bottle, the sealing cover is lifted to the bottom surface of the sealing cover and is slightly higher than a bottle opening of the sampling bottle, so that the oil sample overflows but does not come into contact with the bottom surface of the sealing cover and the overflowing of the oil sample is maintained for 3-8S; (4) the clean cover with the glass pipe is taken away, and then the clean sealing cover of the sampling bottle covers the sampling bottle, thus finishing the sampling. The sampling method provided by the invention has the beneficial effects that the operation is simple, the ultrahigh-pressure insulating oil can be sampled conveniently, no pollution is caused in the sampling process of the insulating oil, and further the accuracy of a detection result is guaranteed.