A pre-supporting combined unit suitable for soft rock tunneling comprises a tail unit, a main unit, a cutting portion and a conveying device. A forepoling bar is arranged at the front end of the main unit. The pre-supporting combined unit is technically characterized in that the main unit comprises a main-unit main supporting set and a main-unit auxiliary supporting set which are staggered from rear to front, the main-unit main supporting set comprises a connecting plate I, a supporting oil cylinder I, a longitudinal beam I, a top beam I and a lateral supporting plate I, and the main-unit auxiliary supporting set comprises a connecting plate II, a supporting oil cylinder II, a longitudinal beam II and a top beam II; the tail unit comprises a tail-unit main supporting set and a tail-unit auxiliary supporting set which are staggered from rear to front, the tail-unit main supporting set comprises a connecting plate III, a supporting oil cylinder III, a longitudinal beam III, a top beam III and a lateral supporting plate II, and the tail-unit auxiliary supporting set comprises a connecting plate IV, a supporting oil cylinder IV, a longitudinal beam IV and a top beam IV. The problem that existing tunneling machines are poor in safety and difficult in back operation is solved fundamentally, and the pre-supporting combined unit has the advantages of low equipment wear and stability in moving.