Selective Laser Melting (SLM) is increasingly recognized as a pivotal technology in laser additive manufacturing. However, the optimization of process parameters remains as a key concern for effective SLM production. This study employs a multi-physics approach to quantify how variations in process parameters (specifically laser power, scanning speed, hatch spacing, and scanning strategy) affect the thermal history and thus the attendant microstructure evolution of Ti-25Nb (at.%) alloy during the dual-track SLM process. Simulation results reveal that increased laser power results in greater thermal accumulation, and leads to a larger molten pool volume and coarser grain size upon remelting. Conversely reduced scanning speed enhances remelting and promote cellular growth at the top of molten pool, while higher scanning speed yield rougher melt tracks and finer grains. Notably, hatch spacing significantly influences molten pool dimensions and microstructure morphology, with smaller hatch spacing promoting remelting. Furthermore, the orientations of grains in the second track during zigzag scanning differ markedly from that in the first track. Most importantly, at the boundaries of the remelted molten pool, both the temperature gradient and cooling rate after the second track scanning are reduced compared to the first, resulting in lowered interface velocities and significant alternation in solidification microstructure. This work provides a theoretical foundation for controlling non-equilibrium microstructure and offer novel insights into optimizing process parameters in the SLM of titanium alloys.