Background: To investigate the hidden link between mitochondrial DNA (mtDNA) and cardiac function after cardiopulmonary bypass (CPB), we compared the relationship of plasma mtDNA level and the prevalence of cardiac dysfunction in patients receiving cardiac surgery with CPB.Methods: From September 1st, 2015 to December 1st, 2016, 962 patients who received cardiac surgery with CPB were prospectively comprised in this study. Routine blood test and examinations for blood biochemistry, NT-proBNP and mtDNA were arranged in half an hour when patients were transferred into intensive care unit (ICU). The clinical outcomes related to the cardiac function were determined by at least two senior doctors responsible for ICU. The data were inputted with double check and the inner logic of these data was set to establish the database. SAS 9.4, R 3.2.4 and SPSS 21.0 were used in data analysis.Results: Of the 962 patients (mean [SD] age, 51.6 [12.5] years; 459 [47.7%] male) comprised in this study, 692 patients received cardiac valve surgery, 83 patients received coronary artery bypass grafting (CABG), 58 patients received CABG and cardiac valve surgery at the same time, 70 patients received surgery on ascending aorta, 35 patients received surgery for simple congenital heart diseases and 15 patients received other types of surgery. The prevalence of cardiac dysfunction in this study is 9.25%. The level of mtRNA among patients with cardiac dysfunction and patients without cardiac dysfunction has no statistical significance (t=-1.42, P=0.1565). There was an obvious positive liner correlation between the level of plasma mtDNA and NT-proBNP in patients receiving CPB (r=0.1291, P=0.0386). There was an obvious positive liner correlation between the level of plasma mtDNA and APACHE II score in patients receiving CPB (r=0.2133, P<0.0001).Conclusions: The level of plasma mtDNA was closely related with the cardiac function and illness severity in patients receiving cardiac surgery with CPB.