The invention relates to an inner formwork for pouring a cylindrical member. The inner formwork comprises an arc-shaped assembly and a demoulding mechanism. The arc-shaped assembly is composed of a large arc-shaped plate, a medium arc-shaped plate, a small arc-shaped plate and plug pin rods. The demoulding mechanism is provided with a first lifting rod system and a second lifting rod system. The first lifting rod system of the demoulding mechanism is mounted between the small arc-shaped plate and the large arc-shaped plate, and the other rod ends of two same rod pieces of a strut assembly arein hinged connection with the small arc-shaped plate and the large arc-shaped plate correspondingly; and in the same way, the second lifting rod system of the demoulding mechanism is mounted between the medium arc-shaped plate and the large arc-shaped plate, and the other rod ends of two same rod pieces of a strut assembly are hinged to the medium arc-shaped plate and the large arc-shaped plate correspondingly. The geometric size of the inner wall of the cylindrical member is obtained through hinging and inserting connection of the arc-shaped assembly, when demoulding is needed, the constraintof the plug pin rods is relieved firstly, then struts are closed through lifting rods, the small arc-shaped plate and the medium arc-shaped plate are rotated around hinging points, the inner formworkis shrunk, and accordingly demoulding is completed smoothly.