The invention provides an underground diaphragm wall groove section and an underground diaphragm wall, and relates to the technical field of underground engineering. The underground diaphragm wall groove section comprises a steel reinforcement cage, a sand prevention layer and a groove; the steel reinforcement cage and the sand prevention layer are arranged in the groove; the extension direction of the steel reinforcement cage is the same as that of the groove; and the sand prevention layer is arranged between the steel reinforcement cage and a silt soil layer. The viscosity of the silt soil layer is poor and a foundation is not stable, so that the probabilities of collapse and neck shrinkage are high. With the adoption of the sand prevention layer, soil bodies on the both sides can be prevented from entering into the steel reinforcement cage, so that the situation that the soil bodies lean on the steel reinforcement cage or intrude into the steel reinforcement cage so that the underground diaphragm wall groove section is subjected to reinforcement leakage or mud clamping is avoided, the stability is further improved and the probabilities of the collapse and the neck shrinkage arereduced.