The longitudinal mode distribution in the self-mode-locked (SML) laser with flexible reflectivity and a tilted mirror is investigated. First, the longitudinal mode characteristics of the SML laser are analyzed theoretically. Then by employing an intracavity etalon, the output mirror with different parameters is used for longitudinal mode manipulation in experimental research. By varying the reflectivity and tilting the output mirror, the longitudinal mode grouping and spectral shifting are observed. The experimental results show that, when the output mirror reflectivity decreases from 95% to 60%, the transmission peak linewidth of the intracavity etalon becomes narrower, and the number of the longitudinal mode selected by each transmission peak decreases from a maximum of 7 to a minimum of 1. When output mirror is tilted by +0.03 deg and −0.03 deg, the longitudinal mode wavelengths redshift and blueshift by 0.02 nm. The experimental results are in good agreement with the theory.