Effective operations of ramjet combustion chambers across a wide range of velocities requires appropriate regulation of both jet and non-jet modes. An analysis was conducted on supersonic velocity-constrained jets in isotopically expanding pipelines to improve the design of combustion chamber profiles and identify the key factors affecting pneumatic regulation. This analysis addresses the lack of systematic research on the main influencing factors of jet aerodynamic control, particularly under high-speed flow conditions inside the combustion chamber. This investigation focused on the effects of environmental conditions and geometry on pipeline flow characteristics and separation. Specifically, back pressure of the flow and geometric expansion angle of the pipeline were independently varied to determine their individual impacts. The results show that increasing the forward inclination and decreasing the geometric laryngeal channel can cause the flow to shift to jet mode, that is, can be reasonable forward inclination design, combined with the regulation of the geometric laryngeal channel during the combustion chamber work to achieve the change from non-jet mode to jet mode.