The airflow structure of enclosure directly affects the spread of COVID-19 and is also closely related to indoor air quality, thermal comfort of personnel, and building energy consumption. Although a large number of studies on the airflow field have been conducted under mixing and displacement ventilation with a single air inlet in rectangular rooms. However, to the best of the authors knowledge, only a limited number of studies have dealt with airflow structures in a multi-slot ventilation enclosure with opposed jets. Therefore, this paper uses PIV to study the velocity and entropy of unstable airflow field in a multi-slot ventilation enclosure with opposed jets under isothermal and non-isothermal conditions. And this paper presents due to the collision of the jets to form two large-scale eddies, the airflow field structure being unstable. In the region without air supply inlets and exhaust outlets, a large-scale vortex is formed in the airflow field, resulting in high information entropy of the flow field. The thermal plume suppresses the large-scale flow field structure and increases the small-scale flow field structure.