Objective To investigate the mechanism of caspase recruitment domain‐containing protein 9 (CARD9) in the early stage of acute pancreatitis(AP) .Methods Peripheral blood mononuclear cells (PBMC ) of 49 AP patients (33 mild acute pancreatitis (MAP ) patients and 16 severe acute pancreatitis (SAP) patients) were collected on the Day 1st ,3rd and 5th of hospitalization .Twenty healthy volunteers were enrolled in control group .The expression level of CARD9 ,B‐cell lymphoma(Bcl)‐10 ,p38 mitogen‐activated protein kinase (MAPK ) and p65 nuclear factor Kappa B (NF‐κB ) in PBMC of AP patients were detected by Western blotting .The co‐localization ,expression and binding between CARD9 and Bcl‐10 in PBMC of control group ,SAP group and MAP group on the Day 1st hospitalization were determined by cell immune‐fluorescence staining and co‐immuno precipitation method .Single factor analysis of variance and Mann‐Whitney test were performed for data comparison between groups .Pearson method was used for correlation analysis .Results The results of Western blotting indicated that the expression of CARD9 and Bcl‐10 in PBMC of SAP group on the Day 1st ,3rd and 5th of hospitalization (1 .12 ± 0 .05 ,1 .03 ± 0 .03 and 1 .01 ± 0 .01 ;1 .74 ± 0 .08 ,1 .72 ± 0 .10 and 1 .69 ± 0 .11) were all significantly higher than those of control group (0 .33 ± 0 .10 and 1 .02 ± 0 .11) and MAP group (0 .71 ± 0 .02 ,0 .55 ± 0 .06 and 0 .25 ± 0 .07 ;1 .15 ± 0 .03 ,1 .09 ± 0 .07 and 1 .01 ± 0 .04) ,and the differences were statistically significant (F= 35 .76 and 18 .20 ,all P< 0 .05) .The expression of p38 MAPK in PBMC of SAP group on the Day lst ,3rd of hospitalization (1 .88 ± 0 .08 、1 .68 ± 0 .11) were significantly higher than those of MAP group on the Day 1st ,3rd ,5th (0 .86 ± 0 .08 ,0 .77 ± 0 .10 ,0 .73 ± 0 .20) and healthy control group (0 .58 ± 0 .24 , F= 7 .24 ,all P < 0 .01) .The expression of p65 NF‐κB in PBMC of SAP group on the Day 1st ,3rd of hospitalization (1 .64 ± 0 .02 ,1 .55 ± 0 .03) were significantly higher than those of MAP group on the Day 3rd ,5th (1 .06 ± 0 .14 ,0 .87 ± 0 .20) and healthy control group (1 .17 ± 0 .13 ,F= 4 .51 ,all P< 0 .05) .The results of immune‐fluorescence staining indicated that CARD9 and Bcl‐10 co‐localized in nucleus .The results of co‐immuno precipitation showed that the binding degree between CARD9 and Bcl‐10 of SAP group was significantly higher than that of control group and MAP group . Pearson correlative analysis suggested that the level of p65 NF‐κB and p38 MAPK in PBMC of AP patients were positive correlated with the expression of CARD9 (r= 0 .692 and 0 .834 ,both P< 0 .01) .Conclusion CARD9 is positive correlated with NF‐κB and MAPK , which indicates CARD9 induced inflammatory cytokines by activating NF‐κB and MAPK signaling pathways in AP .