Sulfur mustard (SM) is a notorious blistering chemical warfare agent. Rapid field screening for trace SM is of vital significance for the detection of antiterrorism and timely treatment. Here, a visual assay for SM was constructed on the basis of its inhibition for the G-quadruplexes/hemin DNAzyme. Specifically, multiple guanine (G)-rich single stranded oligonucleotides (ssODN) named S1 (80% of G in the total bases), i.e., the precursor for G-quadruplex, which could oxide tetramethylbenzidine (TMB) to its green product, were conjugated on the nonfouling polymer brush grafted magnetic beads (MB@P(C-H)). SM could specifically alkylate the N7 and O6 sites of G in the S1; thus, it failed to form the DNAzyme based signal reporter. It was demonstrated that the nonfouling P(C-H) interface on the magnetic bead (MB) could protect the conjugated ssODN from nuclease degradation, thus ensuring its well sensing performance in complex samples. Under the optimized conditions, this method achieved good sensitivity and selectivity with a limit of detection (LOD) as low as 0.26 μmol L-1, and the recoveries ranging from 86% to 117% were obtained for different SM spiked real samples. Above all, this method combining low cost and ready operation could be suited for rapid field SM screening in a wide range of environmental matrices.