1. A composition comprising an FGF21 molecule covalently attached to the antigen binding center of an antibody or antigen binding fragment thereof via a linker. 2. The composition according to claim 1, where the linker is covalently attached to the FGF21 molecule through the side chain of the amino acid binding residue at position 79, 125 or 129 in accordance with the numbering of SEQ ID NO: 1.3. The composition of claim 2, wherein residue 129 is cysteine and the linker is covalently attached to the thiol group of cysteine 129.4. The composition according to claim 1, where the linker contains the formula X-Y-Z； where X is a biocompatible linking chain comprising any atom selected from the group consisting of C, H, N, O, P, S, F, Cl, Br, and I, and may contain a polymer or block copolymer, and is covalently linked to a linking moiety, where the linker is linear, Y is a possibly present recognition group containing at least a ring structure； and Z is an attachment moiety containing a covalent bond to an amino acid side chain in an antigen binding active center of an antibody. The composition according to claim 4, where Y has a possibly substituted structure: where a, b, c, d and e independently represent carbon or nitrogen； f represents carbon, nitrogen, oxygen or sulfur； 6. The composition of claim 5, wherein Y is phenyl. The composition according to claim 4, where Z has a formula covalently attached to the antibody, and q = 1 or 2.8. The composition according to claim 7, where q = 2.9. The composition of claim 4, wherein X is a group containing three components； Xp-Xs-Xy, where Xp is a group specifically adapted so as to be able to join the side chain of the binding o1. Композиция, содержащая молекулу FGF21, ковалентно присоединенную к антигенсвязывающему центру антитела или к его антигенсвязывающему фрагменту через линкер.2. Композиция по п.1, где линкер ковалентно присоединен к молекуле FGF21 через боковую цепь связывающего остатка по аминокислоте в положении 79,
