Magnetic resonance imaging is widely used to identify and monitor thrombi in grafted vessels following coronary artery bypass grafting surgery (CABG). We produced a biosensor, P1Cm-SPIO-Cy5.5, composed of P1Cm peptide, superparamagnetic iron oxide nanoparticles (SPIO), and polyethylene glycol (PEG), for use in MRI thrombus imaging. Activated platelets induced by adenosine diphosphate were used for combination tests in vitro. A rat model of common carotid artery thrombosis was used for anti-thrombus experiments in vivo. P1Cm-SPIO-Cy5.5 remains stable in vitro and has good anticoagulation capacity. It can bind specifically to activated platelets and thrombi. In rats that underwent bypass surgery, P1Cm-SPIO-Cy5.5 could detect and label thrombi over a long period, and prevent thrombosis in grafted vessels. P1Cm-SPIO-Cy5.5 improved cardiac function in rats following CABG surgery. P1Cm-SPIO-Cy5.5 is a potential sensor for use in MRI for the early diagnosis and prevention of thrombosis after CABG surgery.