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The sensitive, point-of-care detection of acute myeloid leukemia (AML)-derived exosomes is essential for non-invasive diagnosis and therapy monitoring, but remains technically challenging. Here we report a colorimetric biosensor based on a dual -aptamer-initiated catalytic cascade, where simultaneous recognition of exosomal markers CD63 and PTK7 by engineered aptamers triggers an in situ rolling circle amplification (RCA) reaction. The resulting RCA product templates the assembly of a metal-organic framework (MOF) nanozyme, which acts as an efficient peroxidase mimic to generate a colorimetric readout. This integrated "recognition -amplification-transduction" nanoarchitecture achieves a detection limit of 0.1 ng/mL for PTK7-positive exosomes and successfully discriminates patient serum samples from healthy controls as well as AML-bearing mouse models from healthy mice, with consistent results obtained via UV-vis spectroscopy, visual colorimetry, and smartphone-based grayscale analysis. By synergizing nucleic acid nanotechnology with nanozyme catalysis, this work provides a rapid, equipment-free visual platform for exosome-based liquid biopsy in AML management.