In order to improve the plasma density and the ionization rate of working medium gas, the helical wave generated by helical antenna is used to excite Ar plasma, and the characteristics of ion density and electron temperature of plasma are analyzed by RF compensated Langmuir probe. The experimental results show that with the increase of air pressure and power, the helicon plasma enters the helicon discharge mode in advance. Under the pressure of 1.0 Pa, when the RF power reaches 400 W, the plasma discharge enters the helicon discharge mode, and the plasma density in the extended region exceeds 1e18 m-3. The electron density is the highest in the center of the discharge tube and gradually decreases along the radial direction. The research results will provide the basis and experience for the large volume H2 helicon plasma.