Rare B meson decays offer an opportunity to probe a light hidden boson. In this work we explore a new channel () followed by a cascade decay of into an invisible (neutrino or dark matter) or charged lepton pair (). The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with . For the first time, we calculate at the leading power of and . Confronting with the strong constraints from semi-invisible decays of B meson, we find that the branching ratio for can be larger than its Standard Model background prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for can also be sizable when the corresponding flavor violating coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of and are severely constrained by the experimental measurements.