To develop safe and effective macromolecular MRI contrast agents, a macromolecular contrast agent (mCA) containing biocleavable disulfide bonds in the main chain and oligolysine in the side chain is prepared, and its applicability as a MRI contrast agent is demonstrated both in vitro and in vivo. This brush-like mCA possesses a high T1 relaxivity (11.8 mM(-1) s(-1)), up to 3 times higher than the commercial Gd-DTPA (4.2 mM(-1) s(-1)), along with very low toxicity as determined by WST assay and histological analysis. Meanwhile, the disulfide bond can be broken under appropriate reducing conditions, followed by degradation into small fragments. Furthermore, the mCA is functionalized with folic acid to improve the target specificity. In vivo experiments show that FA-labeled mCA can efficiently enhance the resolution between the tumor and surrounding tissues compared to the mCA without FA. This study may provide helpful insights for the further development of sensitive and biocompatible MRI probes.