Abstract: In nuclear reactor safety, coolant loss accidents (LOCA) pose critical risks as steam generated from residual heat rapidly pressurizes the containment. To mitigate this, small modular reactors (SMRs) and boiling water reactors (BWRs) employ suppression pools where steam is directly injected into subcooled water to dissipate energy via direct contact condensation (DCC), thereby preventing overpressure and overheating. As the cornerstone heat and mass transfer phenomenon in such passive safety systems, the coupling interaction between steam bubbles and subcooled liquid in DCC governs both condensation dynamics and thermal convection effects, a fundamental process that also profoundly influences subcooled boiling behavior. Recent advancements in experimental techniques, such as high-speed schlieren imaging, have enabled quantitative analysis of bubble condensation behavior. This study experimentally investigates bubble condensation in subcooled liquid using high-speed schlieren techniques, with a focus on the effects of subcooling. Experimental results reveal that subcooling significantly alters the distribution of thermal boundary layers around condensing bubbles. When the subcooling is less than 30 K, the condensation rate at the gas-liquid interface is low, and the corresponding schlieren images exhibit diffuse distributions near the bubble top and wake regions, with a thin thermal boundary layer of only 0.08-0.1 mm thickness at the bubble apex. As subcooling increases to 30 K, the thermal boundary layer thickens to 0.15 mm, and the condensing bubbles become fully embedded within a region dominated by convective heat transfer from condensed water. Under these conditions, dense schlieren structures with sharp dark-light contrast form around the bubbles, and the influence of thermal convection expands in range and intensifies nonlinearly. Additionally, the experiments confirm that the thermal boundary layer thickness increases significantly with subcooling, remaining on the order of 0.1 mm at subcooling levels below 30 K, while its specific distribution evolves gradually with subcooling. The study conclusively demonstrates that high-speed schlieren imaging is a powerful tool for resolving the multiscale physics of condensation-driven thermal convection. By capturing real-time dynamics of thermal boundary layer evolution and quantifying nonlinear convective responses, this technique provides unprecedented experimental insights into DCC mechanisms. Such findings not only advance fundamental understanding of interfacial heat transfer but also inform the optimization of suppression pool designs in next-generation nuclear reactors, ensuring enhanced safety and operational resilience under LOCA scenarios.