Using a method combining numerical calculation and experimental verification, based on the analysis of jet diffusion characteristics, the structural parameters of plenum chambers for smoke control and ventilation and air conditioning systems are optimized. The influences of the width to height ratio (W/H) of the plenum chamber on the windward side and the chamber length (L) on the flow and resistance distribution characteristics are analysed.The research shows that W/H should be designed in stages according to different airflow confinement (m) values.Under the conditions of 0<m≤0.3 and 0.3<m≤0.6, W/H should be designed as 1 and 5, respectively. The jet expansion angle formula is used to judge the degree of airflow confinement in the chamber, and the optimal value of the chamber length is determined to be the greatest of 3.0-4.0 times the equivalent diameter of the windward surface of the plenum chamber, the critical restricted length of the horizontal jet and the critical restricted length of the vertical jet along the chamber, and the structural design method of a single channel plenum chamber is obtained.