The present invention relates to an indolizine derivative and an application thereof in medicine. In particular, disclosed in the present invention are a novel substituted indolizine derivative represented by formula I, or an isotopically-labeled compound thereof, or an optical isomer, a geometric isomer, a tautomer, or a mixture of isomers thereof, or a pharmaceutically acceptable salt thereof, or a prodrug thereof. The present invention further relates to an application of these compounds in medicine.