Abstract: In order to express and purify the cat interleukin-2（Felis-IL-2） in prokaryotic cells, detect its biological activity and explore the protein crystal structure, The Felis-IL-2 gene was amplified from cat peripheral blood lymphocytes by RT-PCR and inserted into the prokaryotic expression vector pET21 a（+） to construct the recombinant expression plasmid pET21 a-Felis-IL-2. Then the recombinant expression plasmid was transformed into E. coli Transetta（DE3） competent cells, and IPTG induced the expression of recombinant protein IL-2（rFelis-IL-2）. The bacterial fluid was collected for SDS-PAGE analysis, molecular sieve chromatography and ion exchange purification, and its biological activity was detected by cell proliferation assay in vitro. The expression of IFN-γ and granzyme A were determined by qPCR. The growth conditions of rFelis-IL-2 protein crystals were screened by the sitting drop method of gas phase diffusion, and the protein crystal structure was determined by X-ray diffraction. The results showed that the recombinant expression plasmid pET21 a-Felis-IL2 was successfully constructed by double enzyme digestion and sequencing. The full length of Felis-IL-2 was 465 bp, encoding 155 amino acids, including 4 exons and 3 introns. The homology with human IL-2 was 81%. rFelis-IL-2, with a molecular mass of 15 ku, could effectively stimulate the proliferation of peripheral blood lymphocytes and promote the expression of IFN-γ and granzyme A significantly up-regulated. Under the PEG/Ion Screen 2 condition 19^#, Crystal Screen condition 15^# and Index 49-96 condition 85^#, the crystals grew occurred at 4 ℃ and 18 ℃ after about 7 days. It indicated that the prokaryotic expression and purification of rFelis-IL-2 had biological activity, which laid a foundation for its further research and industrial production.