High-power fiber lasers are critical in advanced manufacturing and defense applications, yet their high-power output poses severe thermal management challenges that limit long-term stability and further development. Traditional cooling methods suffer from inefficiency and system complexity. This study proposes an immersed phasechange cooling system utilizing the latent heat of the low-boiling-point working fluid R245fa for efficient heat dissipation. By integrating numerical simulations with experimental validation, the thermodynamic characteristics and structural design were analyzed, leading to an optimized integrated packaging solution. Simulation results demonstrated a temperature rise of only 8 ℃ within 100 s, maintaining stability within the safe operating range of laser components. Experimental tests on an engineering prototype confirmed the system’s effectiveness, achieving a total heat dissipation capacity of 25 kW. The proposed system significantly improves thermal efficiency and reliability while reducing energy consumption and system volume, offering an innovative solution for advancing high-power laser technology and broadening its applications with substantial engineering value.