A thermally activated delayed fluorescent molecular material, a synthesizing method therefor, and an electroluminescent device are provided. The thermally activated delayed fluorescent molecular containing an indenobenzoselenoheteroaromatic ring donor is synthesized, so that an electron donating ability of the donor is increased, and an non-radiative transition rate is effectively inhibited, thereby increasing photo-luminescence quantum yield (PLQY) of the molecule; and increasing the twist angle between the electron donor and the electron acceptor. Meanwhile, electron cloud overlapping between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is reduced, thereby obtaining a smaller ΔEST value. The organic electroluminescent device adopts the thermally activated delayed fluorescent molecular material prepared according to the synthesizing method, and thus has high luminous efficiency and long service life.