Objectives To determine the myocardium-protecting effect of medium hypothermal blood cardioplegia (MHBC); further demonstrates that the optimal temperature between these hypothermal and normothermic can overcome the disadvantages;and thus discovers a more effective myocardium protecting method. Methods Section 1: 14 mongrel dogs (15-20 kg) were randomly divided into two groups: experimental group and control group; cardialpulmonary bypass was conventionally instituted,moderate hypothermia blood cardioplegia was used in experimental group, Blood samples from right atrium taken for examination of lactate dehydrogenase (LDH)creatine kinase(CK-MB) and Topin I(cTn-I). specimens of left ventricular subendocardial myocardium were biopsied to observe changes of ultrastructure.Section 2:24 patients were randomly divided into two groups and both groups received two types of treatment (same as Section 1) after aorta cross-clamp (ACC). Biochemical index and Clinical observation were caculated as the indicators. Results In the experimental research, LDH, CK-MB, cTn-I were found increased after reperfusion in both groups, but the extent of changes in experimental group is tiny (statistical difference). Compared with control group the effect of MHBC on cardial function is little; the ultrastructure. of cardiac muscle has no obvious change. In the clinical research, compared the test results of venous blood drawn before CPB, after beating recovery and after CPB, CBC perfused group (LDH, CK-MB leak age and cTn-I value increase.) compared with MHBC perfused group had no remarkable difference (P＞0.05), but at the result of clinical observation: MHBC perfused group had red, soft hearts after cardiac arrest but CBC perfused group had pale, spasmatic hearts after cardiac arrest; 0 case in MHBC perfused group and 3 cases in CBC perfused group had twitch-removing beat recovery; 11 cases in MHBC perfused group and 3 cases in CBC perfused group recovered sinus heart rhythm after surgery; average consumption of lidocaine was 16.67 (±55.28) mg for MHBC perfused group and 108.33(±82.65) mg for CBC perfused group (P＜0.01) after surgery; 4 cases in MHBC perfused group and 11 cases in CBC perfused group suffered arrhythmia after surgery. Conclusions As a new myocardium-protecting method, MHBC perfusion in combination with natural body temperature drop of CPB is worth clinical dissemination and application.