The invention discloses a gene for improving the chlorophyll content of plants and the oil content of rape and an application of the gene. The gene is a BnCHS gene from rape and has a nucleotide sequence as shown by SEQ ID NO.1; the amino acid sequence of the protein coded by the gene is shown by SEQ ID NO.2. A yeast complementation experiment proves that the protein coded by the gene has activity of long-chain acyl CoA synthetase; the analysis of expression profiles in the rape crop shows that the gene is mainly expressed in leaves of seedlings and is mainly expressed in parts with exuberant lipid synthesis of flowers and the like in mature plants; the chlorophyll content of leaves of tobaccos and cabbage type rapes and the oil content of yeasts and cabbage type rape seeds are improved and the biomass of the cabbage type rapes is increased, which are discovered by improving the BnCHS expression quantity through a genetic engineering technology. By adopting the gene disclosed by the invention, the protein coded by BnCHS has functions of improving the chlorophyll content of the cabbage type rapes and improving the oil content of the cabbage type rape seeds.