The High Temperature Gas-cooled Reactor (HTGR) designed by Tsinghua university is under development in China. The electrical equipment in Pressure Vessel, such as magnetic bearings and helium circulator are operating in the high pressure helium environment. Design research of insulation property of the electrical device in helium under high pressure is necessary. Nevertheless, it is challenging to investigate by experimental technique. We propose a similarity law, converting the high pressure to a lower pressure to simplify the experimental conditions. Similarity theory of gas discharge is that two geometrically similar gaps with scaling coefficients of k , based on the same product number of pressure p and the gas length d , have the similar discharge characteristics. We research the validity of discharge similarity theory by simulation. A fluid model of direct-current discharge in helium atmosphere was established, referencing experimental results. And the discharge models of gaps were solved by finite-element method respectively. Four geometrically similar gaps were designed, the prototype gap is 2cm long and operating at a pressure of 600Pa while the pressure of three similar gaps are 300Pa, 200Pa, and 120Pa, and corresponding to the length of gaps are 4cm, 6cm and 10cm, with scaled-down factor k of 2, 3, and 5 respectively. The simulation results show that as long as the scaled-down factor of pressure k is less than 5 and the reduced length relation meets the condition p 1 d 1 = p 2 d 2 , the discharge characteristics of two geometrically similar gaps are similar. As a result, it is realizable to predict the insulation property of electrical device in helium with similarity law.