The patent abstract shows that the present invention relates to the field of electrochemical energy storage technology and discloses an electrolyte additive for vanadium flow batteries. The additive comprises the following components in weight percentage: modified crown ether 80-90%, which includes a basic 18-crown-6 ether and introduces hydrophobic side chains and functional groups; 10-20% of graphene composite materials, in which graphene is surface modified by silane coupling agents; The hydrophobic side chain is selected from alkyl chains, and the functional group is selected from amino and carboxyl groups; The alkyl bromide is selected from C10-C18 alkyl chains to introduce hydrophobic side chains; The graphene composite material comprises a mixture of unmodified graphene and modified graphene. The electrolyte additive of the all vanadium flow battery of the present invention achieves higher electrolysis efficiency, lower internal resistance, more stable charge discharge cycles, and longer battery service life.