The grab type slotting machine for underground diaphragm wall proposed by the invention includes a host machine, a tripod assembly is fixed on the top of the host machine, a main winch is arranged on the tripod assembly, the main winch is located above the host machine, a steel wire rope is arranged on the main winch, one arm assembly is fixed on one side of the host machine, and two arm assemblies are installed on the top of one arm assembly, A roll is arranged on the arm assembly