As a High Power Microwave(HPM) driver, in order to achieve minimization and lightweight, a Marx generator is usually immersed in SF6 or a mixture which contains it. Raising the gas pressure, which would improve the insulation, will facilitate the reduction of the Marx volume. Usually, an external trigger is utilized to ensure the erection of Marx generator. For simplicity and convenience, an attempt of discarding the trigger and erecting the Marx only by self-break spark gaps is made. A 14-stage Marx is established and experiments on that show it is very difficult to erect a Marx when pressure is higher than 0.16 MPa. By analyzing the erection process and optimizing the shape of electrode in spark gaps, a serial of experiments are conducted，and the results indicate that the erecting probability is improved. Finally, a compact Marx generator, working in 0.25 MPa SF6 without trigger，is established，and a 7 GW,70 ns output is obtained on a 45 Ω load.