Compared with the two-electrode gas spark switch, the three-electrode gas spark switch has high controllability, low working voltage and small jitter, so the three-electrode gas spark switch is widely used in pulse power technology. The discharge of gas spark switch is high pressure gas discharge, which is characterized by high electron collision frequency (10 ¹²Hz), small mean free path (10 ^–6m), short breakdown time (10 ^–9s), and complex physical process (including the secondary electron emission, the generation of seed electrons, the space charge effect and various collision processes between electrons and nitrogen molecules, etc). At present, it is difficult to quantitatively describe the breakdown process of the three-electrode gas switch, and the detailed theoretical research is lacking. Therefore, the breakdown mechanism of atmospheric pressure nitrogen spark switch, including two-electrode and three-electrode, is studied theoretically and numerically in this paper. The purpose of this study is to compare the simulation results of the two different gas spark switches, and obtain the characteristics of stream breakdown in different gas spark switches. Firstly, the numerical simulation and theoretical analysis of two-electrode gas spark switch are carried out. According to theoretical and numerical calculation, it can be found that for the plate-plate two-electrode switch, the stream breakdown cannot be generated under low voltage (less than 6.3 kV), while under high voltage (more than 6.3 kV), first the anode-directed streamer is formed, and then the cathode-directed streamer is created. In addition, the simulation results show that the plasma generated by the trigger can effectively reduce the breakdown voltage. Finally, the three-electrode gas spark switch is studied theoretically and numerically. It can be seen that in the breakdown process of the three-electrode gas spark switch, the breakdown first occurs between the trigger and the insulator, and then this plasma channel expands to the anode and cathode, finally forming the arc channel between the anode and the cathode. Under the calculation conditions in this paper, if the cathode-trigger and the anode-trigger are required to break down simultaneously, the applied voltage between the cathode-trigger should be greater than 1.18 kV, while the applied voltage between the anode-trigger should be greater than 3 KV. When the field emission of the trigger is considered, the breakdown threshold can be significantly reduced.