The invention provides a tool type dado of a cast-in-place pile of a soft soil foundation and a construction method for the dado. The tool type dado of the cast-in-place pile of the soft soil foundation comprises a cylindrical steel tube; the inner diameter of the steel tube is larger than the outer diameter of an original pile; four groups of steel channels are fixedly arranged on the inner wall of the steel tube in the length direction of the steel tube and are uniformly distributed on the cross section of the steel tube in the circumferential direction of the steel tube; a double-clamp vibration pile hammer is arranged at the upper end of the steel tube; the upper end of the steel tube is fixed to the bottom of the double-clamp vibration pile hammer in a clamped mode; and a steel wire rope is hung at the upper end of the double-clamp vibration pile hammer. The method has the advantage that the problems that when a pile extension part of the cast-in-place pile is located in a silt soil layer, construction cannot be carried out by adopting a conventional manual hole digging pile extension method, but the cost is too high by adopting a mode that manual hole digging construction is carried out after the soft soil layer is solidified and treated can be solved. The invention belongs to the field of soft soil foundation construction.