Herein, we synthesized two isostructural supertetrahedral T3 cluster-based chalcogenide compounds by an ionic liquid-assisted precursor technique, namely [Bmmim]6In10Q16Cl4∙(MIm)4 (Q = S (In-S), Q = Se (In-Se), Bmmim = 1-butyl-2,3-dimethylimidazolium, MIm = 1- methylimidazole). The two compounds consist of a pure inorganic discrete supertetrahedral [In10Q16Cl4]6- T3 cluster and six charge-balancing [Bmmim]+ anions. The T3 clusters could be highly dispersed in dimethyl sulfoxide (DMSO), exposing more photocatalytic active sites, which makes the highly-dispersed In-Se cluster exhibit ~5 times higher photocatalytic H2 evolution activity than that of the solid-state under visible light irradiation. Comparatively, the photocatalytic performance of the highly-dispersed In-S cluster is only slightly higher than that of the solid state, as its inferior visible-light absorption capability limits the effective utilization of photons. More importantly, through tracking the photogenerated carriers dynamics of highly-dispersed T3 clusters by ultrafast transient absorption (TA) spectroscopy, we found that the photogenerated electrons in the In-S cluster would suffer a rapid internal deactivation process under illumination, whereas the photoexcited electrons in the In-Se cluster can be captured by its surface active centers that would effectively reduce its photogenerated carrier recombination, contributing to the significantly enhanced photocatalytic activity. This work enriches the species of highly-dispersed metal-chalcogenide nanoclusters and firstly investigates the relationship between the structures and photocatalytic performances of nanoclusters by ultrafast excited-state dynamics, which is expected to promote the development of atomically precise nano-chemistry.