Formwork for guaranteeing flatness of side face of preliminary bracing middle partition wall soil body in pipe gallery underground excavation construction and using method of formwork

The invention discloses a formwork for guaranteeing the flatness of the side face of a preliminary bracing middle partition wall soil body in pipe gallery underground excavation construction and a using method of the formwork. The formwork comprises an upper face plate, a lower face plate, a reinforcing plate, inserting heads, inserting openings, face plate feet, face plate foot fixing buckles, fixing grooves, lantern rings, lantern ring fixing rods and lantern ring fixing rod heads. During use, a face plate foot fixing structure is pre-buried into a soil body at the hole bottom of a to-be-excavated chamber, the lantern ring fixing rods are fixed to the corresponding positions according to the positions of the lantern rings, and it is confirmed that all the lantern ring fixing rod heads are located in the same vertical plane. Firstly, the lantern rings of the lower face plate are sleeved into the corresponding lantern ring fixing rod heads, and then the face plate feet are inserted into the fixing grooves; th