MiRNA-21 is highly expressed in almost all tumors, and abnormal miRNA-21 content can be used as an early diagnostic marker for tumor. G-quadruplex-hemin DNAzyme can catalyze the oxidation of 2, 2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)（ABTS）to ABTS + by H 2 O 2 , and based on chain replacement, a visual detection sensor with simple operation and fast response for miRNA-21 was constructed. After optimizing nucleic acid sequence, reaction temperature and time, measuring maximum UV absorbance of ABTS + with different concentrations of miRNA-21 and calculating the SBR with the blank sample, the concentration-SBR curve was drawn. It was found that the lowest concentration to detect was 0.1 μmol/L, when the concentration of miRNA-21 was within the range from 0.1 to 1.2 μmol/L, there was a good linear relationship between SBR and miRNA-21 concentration with the linear equation being y=0.834 9x+ 0.898 3，the linear correlation coefficient being R 2 =0.995. Meanwhile, this colorimetric biosensor was simple in design and operated with high selectivity for detecting miRNA-21.