The invention discloses an I group 11d serotype fowl adenovirus strain, an inactivated vaccine and a preparation method of the inactivated vaccine. The I group 11d serotype fowl adenovirus strain is named as fowl adenovirus RC18, and the virus strain is preserved in the China General Microbiological Culture Collection Center, and the preservation number is CGMCC No. 17404. Experiments prove that the I group 11d serotype fowl adenovirus RC18 strain isolated by the invention has a high titer, strong proliferation ability, good immunogenicity, and matches with epidemic strains. an inactivated vaccine prepared from the I group 11d serotype fowl adenovirus RC18 strain is used to immunize experimental chickens, the experimental chickens can resist infection of the chicken flock caused by clinical epidemic I group fowl adenovirus (11d serotype), the immune response is generated fast, the attack protection rate is high, and no local or systemic adverse reactions caused by the vaccine do not occur, and the safety and stability are good. The invention provides an effective technical means for the prevention and treatment of chicken inclusion body hepatitis.