PA-1Cr18Ni9Ti joints were welded by fiber laser. The thermal process at the interface during the welding process was calculated to analyze its effect on the plastic pyrolysis. Thus the welding heat input was optimized. The calculated results were verified by experiments, in which a stable welded joint of PA-1Cr18Ni9Ti was obtained. In addition, the interfacial connection mechanism was also studied. The results show that the pyrolysis of the PA plastic during the welding process is related to temperature and rate of temperature increase. When the heat input of laser welding is 77.8 J·mm -1 (laser power is 700 W, welding speed is 9 mm·s -1 and defocus amount is 70 mm), the PA plastic can be fully melted and spread at the interface while reducing the pyrolysis, ablation and carbonization. Chemical bond of Cr—O—C can be formed at the PA-1Cr18Ni9Ti interface, which is benefit for joint strength.