To construct the eukaryotic expression plasmid containing the p55 gene fragment of Pneumocystis and to investigate the efficient expression in COS-7 cells, the gene fragment conaining the whole length of p55 gene was used as template to amplify this fragment with PCR and the amplified fragment was then cloned to vector pGEM-T. After enzyme digestion, p55 gene was cloned to the eukaryotic expression vector pcDNA3.1(+) to construct the plasmid pcDNA3.1(+)-582. This plasmid was then transfected to the eukaryotic expression cells COS-7 and PCR and SDS-PAGE assays were used to confirm the presence of target protein in these cells. In these ways, the eukaryotic expression vector for the p55 gene of Pneumocystis of rats was successfully constructed and expressed in COS-7 cells, thus providing the basis for further studies on the nucleic acid vaccine.