Abstract: Microwave coaxial resonators are widely used to generate atmospheric pressure plasma jets which have been applied in many fields such as biomedical treatment and material synthesis. The power threshold of microwave discharge plasma sources under atmospheric pressure is a key issue affecting its wide application. A 2.6 GHz microwave-excited plasma jet source is designed in this paper. Its return loss is 13.99 dB and the working gas is Ar. Under atmospheric pressure, a purple filamentary jet is produced. The breakdown power is 41.1 dBm(12.9 W) and the maintenance power is only 20 dBm(0.1 W). Furthermore, the relationship between the length of the plasma jet and the gas flow rate and input power is explored based on the experimental observation and the simulation results of ANSYS Fluent. It is found that the jet length elongates with the increase of gas flow rate under laminar flow state, and vice versa under turbulent flow state. But the jet length increases monotonously with the input power under different gas flow rates. At the same time, it is found that the plasma does not disappear when the gas flow is stopped; however, the air plasma near the coaxial tip is induced to maintain the ionization state. On this basis, we propose a method of using Ar gas or He gas as the pilot gas to assist other gases to generate cold plasma jets. Finally, CO₂, N₂, O₂ plasma jets can be realized without providing high microwave power.