Calcineurin (CN) is the only protein phosphatase known to be under the control of calcium (Ca2+) and calmodulin (CaM). The enzyme consists of two subunits, the catalytic A subunit of 61 ku (CNA) and a regulatory B subunit of 19 ku (CNB). In this study, we used PCR amplication to construct a truncation consisting of only the CNA catalytic domain. The truncation was induced by IPTG and expressed in E. coli. PNPP was used as a substrate to study the phosphatase activity of the CNA catalytic domain. The findings show that its activity is 20 times greater than CNA in the presence of CNB and CaM. The optimum reaction temperature for the CNA catalytic domain protein is 40℃, and the optimum reaction pH value is 8.0. Mn2+ is still an effective activator for the CNA catalytic domain, but its activity is not controlled by Ca2+. In the presence of 6 mmol/L Mg2+, adding either Ca2+ or EGTA did not change the activity of the CNA catalytic domain.