Objective: The flame spread rate, burning rate, and heat release rate are the key aspects of flammability, which determines the fire development process and the intensity of the heat release. The burning characteristics of a solid fuel strongly depend on the environmental conditions, such as the oxygen concentration, flow rate, and ambient pressure. Most studies have focused on the flame spread rate, and only a few have focused on the burning rate, heat release rate, and soot generation characteristics. When the burning rate of solid materials exceeds the smoke point, the distribution of soot within the flame and the volume fraction of soot undergo a large transformation, thus affecting the heat release rate and changing the flame propagation process. In addition, the generation and transport of soot are crucial for fire safety. An urgent need exists to understand the combustion and soot behavior during flame propagation in real fire scenarios. Methods: In this study, flame spread phenomena over a cylindrical polymethylmethacrylate (PMMA) at different airflow velocities have been experimentally studied under microgravity and normal gravity conditions. Microgravity experiments were performed in a drop tower. In microgravity experiments, flame spread in purely opposed flow was observed, and in normal gravity experiments, downward flame spread behaviors in the mixed flow with buoyancy-induced and forced flows were investigated. The airflow velocities used in both experiments were 1-35 cm/s, and the diameter of the solid sample was 2—10 mm. In the normal gravity environment, the variation in the sample mass during the flame spread process was recorded using an electronic balance, and the soot volume fraction inside the flame was tested using the light extinction method. In both sets of experiments, the luminescent flame and the stoichiometric flame contour photographed with the CH filter were recorded. Results: The flame area, which is estimated from the stoichiometric contour of the CH radicals of the flame, shows a good linear correlation with the measured mass burning rate. Meanwhile, the flame area decreases with increasing flow rate in a normal gravity environment, while in a microgravity environment, the flame area increases to a maximum value and then decreases with increasing opposed flow velocity, indicating a nonmonotonic variation trend. The soot formation of PMMA specimens depends on the diameter of the specimen and the flow conditions, and the experiments in normal gravity show that larger specimen diameters and lower flow rates favor soot formation. However, the flow velocities corresponding to the smoke points of PMMA specimens in different gravity environments are quite different. The flow velocities corresponding to the smoke points of specimens in microgravity environments are even lower. In normal gravity, the soot concentration in the flame decreases with increasing flow velocity. In contrast, in microgravity, solid materials have different smoke points, and the soot concentration increases with the convection velocity. Conclusions: The fuel burning rate and soot formation depend on the airflow velocity. The relationship between the flame area and the burning rate is independent of the fuel smoke point. Because of the variation in the flow condition, the resident time and oxidization time become different, resulting in variation in the soot formation characteristics.