An enhanced three-dimensional transient Thermal-Fluid-Dilution model, integrating a sub-model that accounts for surface tension, is proposed to investigate the unconventional thermal behavior and dilution induced by the surface-active element (sulfur) during the laser joining process of 304SS with higher sulfur content and pure nickel with lower sulfur content. Surface tension is modeled as a function dependent on local temperature and the concentration of the surface-active element (sulfur) at the free surface. The temperature and sulfur-dependent surface tension initially generate an unconventional driving force, subsequently leading to the transitions of heat transfer, flow pattern, and species dilution. Specifically, the inclusion of the sulfur effect results in a redistribution of larger surface shear stress along the transition boundary, causing unconventional flow behavior. The temperature coefficient of surface tension (TCST) is the coexistence of TCST (-) and TCST (+), leading to a novel flow pattern with combined inward-outward flows. Moreover, the anomalous flow exhibits observed phenomena such as the rotation of the major axis (RA) and center line shift (CLS) within the melt pool, showing more reasonable simulated melt pool dimensions when accounting for the sulfur effect. Additionally, the dilution model integrated with the unconventional flow pattern presents a more uniform concentration distribution compared to the conventional dilution. Finally, the simulated distribution of alloy elements is validated effectively by experimental data obtained from Energy Dispersive Spectrometer (EDS) analysis.