MicroRNAs (miRNAs) serve as potential biomarkers for many diseases such as cancer, neurodegenerative diseases and cardiovascular conditions. The portable and accurate detection of miRNA is of great significance for the early diagnosis, treatment optimization and prognostic evaluation of diseases. Herein, a photothermal/visual dual-mode assay for let-7a is developed utilizing oxidized 3, 3', 5, 5' - tetramethylbenzidine (oxTMB) as signal reporter. Let-7a efficiently initiates two cascade amplification processes of catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR) on the magnetic microspheres. It is worth noting that the hairpin DNA employed for HCR is labeled with horseradish peroxidase (HRP). Consequently, a substantial accumulation of HRP occurs on the magnetic microspheres and further catalyzes the oxidation of TMB by H