The invention discloses a method for breeding a transgenic plant having an increased single grain number or main grain number. The method comprises the following step of: transferring an encoding gene of a protein a) or b) into a target plant to obtain the transgenic plant which has the increased single grain number, main grain number and/or plump grain number compared with the target plant, wherein a) a protein is represented by the sequence 3 in the sequence list; and b) protein b is derived from the protein a by substituting and/or deleting and/or adding one or more amino acid residues related to the grain number of the plant into the amino acid residue sequence 3 in the sequence list. The experiments show that the transgenic plant obtained by the method under the conditions of water culture, sand culture and soil culture has obviously increased single grain number, main grain number and/or plump grain number compared with the target plant.