<p>Variable air volume systems are widely used in trains as they provide effective energy savings for air conditioning. However, the design phase is usually carried out at a certain fixed airflow, resulting in a difference between the expected and actual flow fields. In this study, we first analyzed the flow field characteristics at variable airflow rates under isothermal conditions, and subsequently established a correlation equation between the airflow and the flow field parameters. The results show that the airflow just entering the compartment conforms to the characteristics of an attached jet, and then, after converging in the middle, conforms to the characteristics of a free jet. The flow field characteristics are significantly correlated with the Reynolds number (<i>Re</i>) and are limited by the small space inside the vehicle. When <i>Re</i>⩾2650, the axial velocities all conform to the classical <i>u</i>_m/<i>u</i>₀−(<i>x/b</i>)′ or <i>u</i>_m/<i>u</i>₀−(<i>y/b</i>)^<i>r</i> law, the sectional velocities conform to the exponential and Gaussian distributions, and the characteristic thicknesses of the jets conform to a linear distribution. Derivation shows that the entrainment suction flow rate is related mainly to the initial flow rate and the air opening width. The results of this study will inform the design of ventilation systems for high-speed trains.</p>