Thermoplastic PMMA has high transparency and chemical stability, and is widely used in automotive doors, windows, lights and other industries. In this paper, carbon black was used as an absorbent to explore the influence of process parameters (carbon black content, line energy density and PMMA thickness) on weld quality (tensile force and weld width) in PMMA laser transmission welding process, and SEM observations were made of weld section morphology with different carbon black content and line energy density. The results indicate that both carbon black content and line energy density initially enhance the welding strength, but excessive levels lead to a decline. In contrast, as PMMA thickness increases, the welding strength also increases, albeit with a gradually diminishing rate of improvement. Notably, all three factors exhibit a positive correlation with weld width. Notably, at an optimal carbon black content of 0.15 wt.% and line energy density of 1.11 J·mm-1, a “ridge-like” cross-sectional morphology is observed, indicating thorough plastic melting and inlaying, thus yielding excellent welding quality.