Catalytic hairpin assembly (CHA) is an enzyme-free amplification method that it has been proved to be useful in signal amplification. In this work, hydroxylamine-O-sulfonic acid (HOSA) is used as an oxide for LumAuNPs chemiluminescence (CL) in mismatched catalytic hairpin assembly (MCHA) for the first time. Functionally, this system consists of two hairpin species, the biotin-labeled hairpin H1 and LumAuNPs-labeled hairpin H2, which are stable and unable to hybridize in the absence of target DNA. However, the MCHA is initiated in the presence of target DNA, which regenerates the target DNA to catalyze another new cyclic reuse and leads to the formation of numerous H1-H2 complexes. In this case, HOSA serves as an oxidizing agent reacted with LumAuNPs, which can generate strong CL. In this fascinating hairpin assembly amplification strategy, mismatched base sequences are designed in hairpin H2 to decrease nonspecific CHA products, which reduce the background signal significantly. Under the optimal experimental conditions, this approach shows high selectivity even toward single-base mismatch and improves sensitivity greatly for target DNA with a linear range of 2.5 fM–1 pM, achieving a detection limit as low as 0.8 fM. The developed method holds a great promise for further applications in early clinical diagnosis and therapy.