OBJECTIVE： To study the mechianism of inhibitory effect of ulinastain on inflammatory factors of patients during cardiopulmonary bypass （CPB） cardiac surgery. METHODS： Totally 40 patients underwent selective CPB cardiac surgery collected from our hospital during Jul. 2012-Jul. 2016 were divided into control group and observation group according to random number table， with 20 cases in each group. Observation group was given 300 000 U of ulinastatin at 2.5 mL/min by intravenous pump within 20 min after anesthesia induction，  then given 700 000 U of ulinastatin at 0.2 mL/min by continuous intravenous pump until operation finish. Control group was given equal volume of normal saline. Blood samples of patients were collected before anesthesia induction （before surgery） and 6， 12， 24 h after CPB （after surgery）， respectively. The expression of miR-155 and its target gene MyD88 in peripheral blood mononuclear cells of each group were detected by Real-time PCR and Western blot. The expression of TNF-α， IL-6 and IL-8 were measured by ELISA. The correlation of miR-155 with the expression of TNF-α， IL-6 and IL-8 in patients of observation group 24 h after surgery were analyzed by using Pearson correlation analysis. RESULTS： Compared with control group， the expression of miR-155 in observation group was significantly increased， while the expression of its target gene MyD88 was significantly decreased； the expression of TNF-α， IL-6 and IL-8 were reduced significantly， with statistical significance （P＜0.01）. The expression of miR-155 was significantly negative correlation with the expression of TNF-α， IL-6 and IL-8 in the patients of observation group 24 h after surgery （P＜0.01）. CONCLUSIONS：Ulinastatin can inhibit the release of inflammatory factors TNF-α，IL-6 and IL-8 by inducing the expression of miR-155， which may be a new mechanism for the anti-inflammatory effect of the drug in CPB cardiac surgery.