A so-called Semi-Active Support (SAS) system was proposed for the primary mirror of the YNAO 2-m Ring Solar Telescope (2-m RST of Yunnan Astronomical Observatory, China), whose primary mirror is distinctively figured in a ring with an outer diameter of 2.02 m and a ring width of 0.35 m. This paper reports the design and tests of the prototype of the pneumatic-based semi-active axial support system of the 2-m RST. The dummy mirror was a solid circular plate glass of 700 mm in diameter and 20 mm in thickness, which was support by nine Pneumatic Force Actuators (PFAs) organized in three groups, each group was regulated by one proportional regulating solenoid valve. Besides, three Displacement Actuators (DAs) were used to actively define the three Degrees Of Freedom (DOFs) of piston and tip/tilt of the dummy mirror. The pneumatic force actuators were to be actively pumped and regulated, meanwhile, the displacement actuators to actively define the primary's position, according to the variation of the elevation pointing of the telescope. The PFA was build with a metal bellow as its cylinder, and a pressing load cell of 200 N capacity with 5 mN resolution was integrated on its output tip. The DA was a step-motor based design with a travel range of 4 mm and a theoretical resolution of 50 nm. Basic technical requirements for the PFAs and DAs were reviewed first. General consideration and configuration of the prototype was elaborated, followed by detailed designs and comprehensive tests of the either type standalone actuator. The prototype was finally tested on a systematic level as well. This prototype study has paved a reasonable way for critical design of the axial support system of the 2-m RST primary mirror.