The lactase gene(lac4) was amplified from the genome of Kluyveromyces lactis through polymerase chain reaction(PCR) and then cloned into Escherichia coli expression vector pET-28a(+),and the lac4 gene was successfully expressed in Escherichia coli21,enzyme activity reached （44.78±2.84）U/mL.The recombinant lactase was purified with affinity chromatography technology,then analyzed by SDS-PAGE,its molecular weight was approximate 122 ku.The optimal reaction conditions of lactase such as temperate,pH were 43 ℃ and 7.0,its half-life was 30 min at 37 ℃.It was very stable on the condition of pH 4.0 to10.0,and can be greatly activated by Ca2+，Zn2+，Fe2+，Co2+，Mg2+，Mn2+,its specific activity was (255.55±2.32) U/mg,the response constant Km was 3.49 mmol/L,the maximum reaction velocity vmax was 4.22 mmol/（min·mg）.