MXenes exhibit a variety of unique electronic, optical, chemical, and mechanical properties. In this work, the nonlinear optical (NLO) properties of Nb4C3Tx are systematically investigated. The Nb4C3Tx nanosheets exhibit saturable absorption (SA) response from visible region to near-infrared region and better saturability under 6 ns pulse excitation than that under 380 fs excitation. The ultrafast carrier dynamics show a relaxation time of ∼6 ps, which suggests a high optical modulation speed of ∼160 GHz. Consequently, an all-optical modulator is demonstrated by transferring the Nb4C3Tx nanosheets to the microfiber. The signal light can be modulated well by pump pulses with a modulation rate of 5 MHz and an energy consumption of 12.564 nJ. Our study indicates that Nb4C3Tx is a potential material for nonlinear devices.