Display omitted •A new XGBoost prediction model for real-time SSC prediction in laser welding.•68.6 % increase rate of the new model from 0.2947 to 0.9383.•Li I at 395.09 nm shows the highest importance followed by Al I at 669.84 nm, Mg I at 518.4 nm and Ar I.•A good linear and positive correlation between the spectrum intensity of Mg I (517.27 nm) and seam strength coefficient. This paper studies the regression prediction of laser welding seam strength of aluminum-lithium alloy used in the rocket storage tank by means of the optical spectrum and extreme gradient boosting decision tree (XGBoost). First, the relationship between the spectrum intensity and the seam strength coefficient is thoroughly investigated through parameters changing experiments using the developed monitoring system of the optical spectrum. Then, the importance of the metal line spectrum, including Al I, Li I, and Mg I, is quantitatively evaluated, and good complementarity between the Random Fores(RF)t and Principal Component Analysis(PCA) is demonstrated. Finally, a novel regression model, e.g., RFPCA-XGBoost is proposed and is compared with other different feature selection methods, tree-based ensemble learning models and grid search parameters optimization, and the comparison results show that among all the methods, the proposed model has the best performance regarding the R2 value, achieving the R2 value of 0.9383.