To obtain the base data for insulation design of the electrical equipment used in the high temperature gas cooled reactor nuclear power plant, an experimental apparatus for testing helium insulation property under high pressure is designed. The apparatus is composed of a pressure vessel, a heating system, an electrical penetration assembly, a vacuum pump, a pressure gauge, a safety pressure valve, a release valve, and a helium bottle. The highest pressure that the vessel can hold is 10 MPa, and in this experiment the safety pressure valve is set to be 8 MPa. The temperature inside the vessel can be heated to 200 ℃ by a heating system. The resolution of the pressure gauge is 1 kPa, and the highest pressure that the gauge can measure is 9.999 MPa. The purity of the helium used in this experiment is 99.999%. The breakdown voltage of helium gas between two parallel plane electrodes is measured by the apparatus under the conditions of 15-20 ℃ and 0.1-7 MPa. The electrodes are made of copper, and their diameters are both 100 mm. The distances between the parallel electrodes are 0.25 mm, 0.35 mm and 0.5 mm respectively. The error of the distance is less than 0.01 mm. The DC voltage between the electrodes is supplied by GPI-735A, a withstand voltage and insulation tester produced by GW Instek corporation. The voltage increases slowly from 0 to 6000 V (highest), until the current more than 0.1 mA is detected. The highest voltage recorded is the breakdown voltage. With other researchers’ experimental data under low pressure, the Paschen equation for helium gas is obtained. It is found that the calculated breakdown voltage for this equation is larger than the experimental result in this paper under high pressure. And the deviation becomes larger as the product of the pressure and the distance increases. Firstly, it is because the ionization coefficient γ in the equation is influenced by the gas pressure. Secondly, because of larger areas of electrodes, worse surface roughness and less electrode distance, under the same product of the pressure and the distance, the breakdown voltages in this paper are less than the ones in other researches, which are the base of the calculated values. The Paschen equation is modified to accord with the values under high pressure. Under high pressure, the Paschen curve is almost a straight line. A linear equation is presented for calculating the breakdown voltages of helium gas under 0.1-7 MPa. And an equation is presented to calculate the slope of the line. The slope is influenced by the collision cross section, the ionization energy and the temperature of the helium. The experimental data also show that the breakdown voltage of helium is far lower than that of air under the same condition. As the pressure increases, the breakdown voltage of helium increases. The value of helium under 3 MPa is equal to the one of air under atmosphere, and the value of helium under 7 MPa is about twice as high as that of helium under 3 MPa. So, it is possible to replace some experiments or tests under high pressure helium by the same operations under atmospheric air.