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A photothermal fiber sensor based on a microfiber knot resonator (<i>MKR</i>) and the Vernier effect is proposed and demonstrated. An <i>MXene</i> Ti₃C₂T_x nanosheet was deposited onto the ring of an <i>MKR</i> using an optical deposition method to prepare photothermal devices. An <i>MXene</i><i>MKR</i> and a bare <i>MKR</i> were used as the sensing part and reference part, respectively, of a Vernier-cascade system. The optical and photothermal properties of the bare <i>MKR</i> and the <i>MXene</i><i>MKR</i> were tested. Ti₃C₂T_x was applied to a photothermal fiber sensor for the first time. The experimental results showed that the modulation efficiency of the <i>MXene</i><i>MKR</i> was 0.02 nm/mW, and based on the Vernier effect, the modulation efficiency of the cascade system was 0.15 nm/mW. The sensitivity was amplified 7.5 times. Our all-fiber photothermal sensor has many advantages such as low cost, small size, and good system compatibility. Our sensor has broad application prospects in many fields. The proposed stable <i>MKR</i> device based on two-dimensional-material modification provides a new solution for improving the sensitivity of optical fiber sensors.