Cardiopulmonary bypass (CPB) is widely used in cardiac operations. However, it remains unclear whether a CPB circuit, which is mainly made of plastics, can release micronanoplastics (MNPs) into the bloodstream. We conducted a prospective observational study involving children undergoing congenital heart disease repair with CPB support. Blood samples were collected before and after CPB and analyzed using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and laser direct infrared spectroscopy (LDIR) in combination with scanning electron microscopy. A total of 22 patients were involved in this study. The Py-GC/MS analysis revealed a significant increase in total MNPs after CPB support (p < 0.0001). Notably, CPB support significantly increased the levels of polystyrene (p = 0.046), polyethylene (p = 0.038), polypropylene (p < 0.0001), polyvinyl chloride (p < 0.0001), and polyamide 6 (p = 0.027). CPB time was positively correlated to MNP exposure (r = 0.43, p = 0.047). Increases in MNP exposure were positively correlated to an increase in white blood cells (r = 0.52, p = 0.013) and neutrophils (r = 0.46, p = 0.029). The LDIR analysis found that the post-CPB count of MNPs was significantly higher than the pre-CPB count (p = 0.015). In conclusion, CPB support significantly increases exposure to MNP in children undergoing cardiac operations. Further investigations are warranted to clarify the long-term health risks of MNP exposure caused by CPB support.