To　enhance　the　strength　of　chiral　light-matter　interaction　for　practical　applications,the　chirality　and　quality　factors(Q-factors)of　current　methods　need　to　be　strengthened　simultaneously.Here,we　propose　a　design　of　photonic　crystal　slabs(PhCs)supporting　chiral　bound　states　in　the　continuum(BICs)of　transverse　electric(TE)and　transverse　magnetic(TM)modes,exhibiting　maximal　chiroptical　responses　with　high　Q-factors　and　near-unity　circular　dichroism(CD=0.98).Different　from　the　past,the　PhCs　we　employed　only　have　reduced　in-plane　symmetry　and　can　support　simultaneously　chiral　quasi-BICs(q-BICs)of　TE　and　TM　mode　with　two-dimensional　ultra-strong　external　and　internal　chirality.Based　on　the　temporal　coupled-mode　theory,two　analytical　expressions　of　CD　of　chiral　q-BICs　response　are　revealed,which　are　consistent　with　the　simulation　results.Furthermore,we　elucidate　these　results　within　the　charge-current　multipole　expansion　framework　and　demonstrate　that　the　co-excitation　of　higher-order　multipole　electric/magnetic　modes　is　responsible　for　near-perfect　CD.Our　results　may　provide　more　flexible　opportunities　for　various　applications　requiring　high　Q-factors　and　chirality　control,such　as　chiral　lasing,chiral　sensing,and　enantiomer　separation.