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Early diagnosis of the colitis-associated colorectal cancer (CRC) is of great significance for improving prognosis and survival rates. However, the clinical used colonoscopy and biopsy methods are invasive and lack of sensitivity at early stage of cancerization. Herein, we present an amplified sensing strategy in the second near-infrared b (NIR-IIb, 1500-1700 nm) window for non-invasive in situ visualization of early cancerization biomarker miRNA-21. A CRC nanosensor composed of Er³⁺doped lanthanide nanoparticles, DNAzyme, and IR820 dye is designed as a NIR-IIb ratiometric luminescence reporter, providing rapid feedback and high sensitive miRNA-21 detection with limit of detection (LOD) of 1.26 pM. This strategy enables a non-invasive detection of colitis-associated cancerization up to ∼4 weeks ahead of the clinically used biopsy analysis, providing a promising alternative for early diagnosis of chronic colitis cancerization and guidance for therapeutic management.