The invention provides a three-dimensional graphene with a nano-micron graded pore structure and a preparation method of three-dimensional graphene. The content of O in three-dimensional graphene is 5wt%-30wt%, the thickness is 20 mu m-10 mm, the specific surface area is 400-1,200 m<2>/g, and electric conductivity is 1-100 S/cm; diameters of nano pores and micron pores of three-dimensional graphene are 10-300 nm and 0.5-50 mu m respectively. The preparation method comprises steps as follows: 1), preparation of a mixed liquid: a graphene oxide solution prepared with a modified Hummers method is subjected to ultrasonic treatment with silicon dioxide for 0.5-3 h; 2), preparation of three-dimensional graphene oxide: the mixed solution prepared in the step 1) is subjected to vacuum filtration, a thin film is obtained and is treated with a reducing agent and an etching liquid, and vacuum or freeze drying is performed; 3), preparation of three-dimensional graphene: three-dimensional graphene oxide obtained in the step 2) is reduced at the temperature of 300-1,000 DEG C for 0.5-20 h in Ar atmosphere. A three-dimensional graphene porous material has stable pores, is high in specific surface area and electric conductivity and has broad application prospect in lithium ion batteries.