Objective To construct a regulatable plasmid containing single chain fusion gene of murine interleukin-12 (mIL-12) which was regulated with mifepristone (RU486) and explore its expression in vitro. Methods The p40 and p35 subunit sequence of mIL-20 were respectively obtained from the plasmid GCp35Ep40PN by polymerase chain reaction (PCR) and they were cloned into pCA14 plasmid after introducing a linker by overlap PCR. The single chain mIL-12 gene was comfirmed by sequencing and subcloned into pRS-17 vector which contains RU486 regulator cassette. The positive clone named pRS-RUmIL-12 was identified by restriction endonuclease digestion and PCR. Lipofectamine 2000 was used to transfect the pRS-RUmIL-12 to HEK293 cells followed by manufacturer’s recommendations. The protein concentration of mIL-12 induced with RU486 in supernatant of the transfected HEK293 cells was measured by ELISA. Results The sequence of single chain mIL-12 what we obtained was the same as the expected result. The results of restriction endonuclease digestion and PCR showed that the RU486-inducible regulatory vector (pRS-RUmIL-12) was successfully constructed. No significant mIL-12 protein concentration in supernatant of HEK293 cells activation was measured without the inducer RU486, whereas higher concentration of the mIL-12 protein was observed in the presence of RU486. The relationship of concentration of the mIL-12 protein and RU486 was positive correlated under definite range. Conclusion A regulatable eukaryotic expression plasmid of mIL-12 single chain fusion gene was constructed, which could be used in the further research of gene regulation and gene therapy.