Laser welding under subatmospheric pressure was implemented, and the characteristics of weld bead were analyzed in terms of its penetration depth, width and heap height. Based on the analysis of behaviors of plasma plume captured by a high-speed camera, the attenuation effect was evaluated for different subatmospheric pressures. In order to understand the extinction of fiber laser, a probe laser was placed passing through plasma plume. And its extinction was obtained by solving Rayleigh approximation equation. In the given condition, while the ambient pressure was reduced to 3 kPa, the welding penetration depth became approximately two times deeper than that in normal atmosphere. To better illustrate the effect of subatmospheric pressure, the plasma plume was assumed to consist of diffusive, condensed and residual plasma plume. And the residual one plays dominant role in the extinction of fiber laser. When the ambient pressure dropped from 101 kPa to 3 kPa, the decent trend of extinction became more obvious.