<p>Using temperature-sensitive phosphorescent materials, particle tracing technology presents a promising avenue to simultaneously obtain temperature and velocity fields in thermal fluids. However, the application of the technique is limited by the poor particle tracking ability of inorganic phosphorescent materials, particularly in low-speed flows due to their high density. To address this problem, this study developed fluid density-matched phosphorescent microspheres. Phosphorescent microspheres with hollow structures were synthesized via emulsion polymerization, which enables them to maintain the temperature measurement functionality while exhibiting favorable fluid density-matching properties and enhanced flow field tracking capabilities. The microsphere size and average density were meticulously controlled by adjusting the stirring time and temperature. The microsphere diameters were 57–120&#xa0;μm, the theoretical average densities were 0.58–3.2&#xa0;g/cm³, and the operational temperatures were 0–200&#xa0;°C. The result of the numerical simulation indicates that the temperature response time of the microsphere was within 1.41&#xa0;ms. Based on the developed microspheres, a temperature–velocity simultaneous measurement method was developed for low-speed thermal fluids. An application demonstration simultaneously measured the temperature and velocity fields in low-speed hot–cold mixed flows. Comparison with thermocouple measurements reveals that the current method can achieve a fluid temperature measurement with an error of 1.575%. The results underscore the efficacy of fluid density-matched phosphorescent microspheres in simultaneously acquiring temperature and velocity fields in low-speed thermal flows.</p> Graphic abstract <p></p>