A mathematical model was established for pressure-flow coupling system in gas flow equipment by mechanism modeling and step-response methods. The performance stability and control rapidity in gas flowmeter testing were improved by a decoupling control, active disturbance rejection control (ADRC). For the multivariable system of gas flow equipment, a decentralized ADRC integrated coupling effects, internal uncertainties, and external disturbance into a total disturbance, which was estimated by reduced-order extended state observer (RESO) and cancelled out by control law. The original coupling system was then decoupled into two subsystems of single-input and single-output and controlled by proportional differential controllers. Stability and stability margin of the ADRC controlled system were analyzed by frequency domain method. ADRC achieved system decoupling, reduced algorithm dependency on mathematical model, and improved system robustness. Simulation and experiment results show that ADRC algorithm has shorter settling time, better decoupling effect, greater disturbance rejecting capability, and more robust performance than PID controller.