Environmentally friendly materials that exhibit high-performance electromagnetic interference (EMI) shielding are extremely necessary. Herein, we fabricated ultrathin Ti3C2Tx (U-Ti3C2Tx) MXene nanosheets (NS) by atomic-layer tailoring the layer thickness of Ti3C2Tx MXene. The U-Ti3C2Tx NS composites with highly efficient EMI shielding effectiveness can reduce secondary reflection, demonstrating its environmentally friendly performance. The U-Ti3C2Tx NS composite with 80 wt% loading exhibits an EMI shielding effectiveness of 58.1 dB at a thickness of 1 mm. Shielding performance analysis of different layer thicknesses shows that electron transport has an important contribution to the EMI shielding performance. Furthermore, the polarization induced by defects and terminal atoms plays an important role in the EMI shielding performance. Based on the electromagnetic (EM) wave response mechanism, a novel approach to effectively tune the EMI attenuation and shielding effectiveness can be achieved by adjusting the local conductive network. These findings will offer an effective strategy for designing environmentally friendly 2D materials with high-performance EMI shielding.