The invention relates to a dehydration methylate of a quinolinone alkaloid derivative and a preparation method and application. The preparation method is characterized by first carrying out strain culture on marine fungi in a strain medium, then carrying out fermentation culture in a fermentation medium, filtering obtained fermentation liquid to obtain thalli and extracting the obtained thalli byan organic solvent; carrying out chromatographic separation on extraction liquid after the extraction liquid is concentrated, concentrating obtained eluent to obtain yellow powder, namely 2-pyrrolyl-3-carboxyl-4-quinolinone; and adding a methylation reagent and a catalyst into acetone solution in which the 2-pyrrolyl-3-carboxyl-4-quinolinone is dissolved for reaction, and then obtaining the dehydration methylate of the quinolinone alkaloid derivative after reaction. The compound of the invention has strong effect on killing cotton aphids, plutella xylostella and heliothis virescens, can be used as a microbial pesticide and has extensive application prospect. In addition, the raw materials of the compound can be produced in large scale.