摘要： The e?ect of the growth of ambient pressures on the penetration of laser welded molybdenum (Mo) alloy was explored. It was found that when ambient pressure rose from 0.1 MPa to 1.8 MPa, the penetration of base metal (BM) was signi?cantly reduced, which was only 17% of that obtained under ambient pressure of 0.1 MPa. Moreover, the mechanism underlying the signi?cant reduction of the penetration of BM was analyzed. At ?rst, by using a high-resolution scanning electron microscope (SEM), the size and the number of nano-sized metallic particles generated during laser welding under di?erent ambient pressures were surveyed. Furthermore, the scattering and absorption of the nano-sized metallic particles for laser energy under di?erent ambient pressures were investigated; afterwards, by applying a high-speed camera and a spectrometer, the transient behaviors and spectral signals of plasmas during ?ber laser spot welding under di?erent ambient pressures were monitored. On this basis, the inverse bremsstrahlung absorption of plasmas for laser energy under di?erent ambient pressures was explored; ?nally, ?ber laser spot welding test was carried out on glass/metal composite samples under di?erent ambient pressures to survey the in?uence of the change of ambient pressure on dynamic behaviors of the molten pool during the welding.