Dicyclic fused heterocyclic compounds, and applications and pharmaceutical compositions thereof

The invention provides dicyclic fused heterocyclic compounds represented by general formula (I), or isomers or pharmaceutically acceptable salts thereof, and applications of the dicyclic fused heterocyclic compounds. Specifically, the dicyclic fused heterocyclic compounds can be used as PI3K inhibitors, so that the dicyclic fused heterocyclic compounds can be used for treating diseases and symptoms caused by malfunction of PI3K-AKT-mTOR signaling pathway. The invention also relates to pharmaceutical compositions which contain the dicyclic fused heterocyclic compound and are used as PI3K inhibitors.