van der Waals (vdW) transition metal oxides have attracted extensive attention due to their intriguing physical and chemical properties. However, primary drawbacks of these materials are the lack of band structure tunability and substandard optical properties, which severely hinder their implementation in nanophotonic applications. Atomic intercalation is an emerging structural engineering approach for two-dimensional vdW materials to engineer the atomic structure and modify the optical properties, thereby broadening their range of applications. Herein, we synthesized tin-intercalated ultrathin α-MoO3 (Sn-MoO3) nanoribbons via chemical intercalation method and then investigated the broadband nonlinear optics (NLO) of stable few-layer α-MoO3 by performing a Z-scan laser measurement and femtosecond-resolved transient absorption (TA) spectroscopy. Sn-MoO3 showed a stable structure of Mo-O-Sn-O-Mo and a shorter relaxation time than pristine MoO3, indicating the accelerated recombination process of electrons and holes. Furthermore, Sn-MoO3 nanoribbons were used as an optical saturable absorber for ultrafast photonics; a highly stable femtosecond laser with a pulse width of 467 fs was generated from a single-mode fiber in the telecommunication band (1550 nm). These results indicate that atomic intercalation is an effective way to modulate the band structure and nonlinear optical properties of α-MoO3, which hold a great potential in the generation of ultrafast mode-locked laser pulses for optical communication technologies.