Abstract: Porcine circovirus type 2 (PCV2) capsid protein is the only immunodominant structural protein of PCV2 for preparing vaccines against PCV2 infection. The protein can be expressed in Escherichia coli and baculovirus/insect cell expression system. However, studies on the expression of the protein in mammalian cells remain elusive. Here, we investigated several factors that determine the transient expression of the PCV2 capsid protein in mammalian cells. The results showed that a 35% efficiency of the PCV2 capsid gene transfection could be obtained by using HEK293F as the host cell and PEI-40kDa as the transfection reagent. The size and morphology of the PEI/DNA complex was determined by the ratio of the two components, and a particle size range of 15—80 nm contributed to a high cell transfection efficiency. A novel PCV2 Capsid(△1-41aa)-Fc(pig) protein (PCFP) was designed from the immune escape virus strain of PCV2b 41513. PCFP was transiently expressed in HEK293F cells cultivated at a 3 L bioreactor, and the expression level reached 3.8 mg/L. PCFP self-assembled into 41 nm virus-like nanoparticles, which effectively stimulated the humoral immune response in mice.