As one of the most important key technologies for future advanced light source based on the energy recovery linac,a photocathode dc electron gun is supported by Institute of High Energy Physics (IHEP) to address the technical challenges of producing very low emittance beams at high average current.Construction of the dc gun is completed and a preliminary high voltage conditioning is carried out up to 440 k V.The design,construction and preliminary HV conditioning results for the dc gun are described.