The invention discloses a positioning device and method for a cylinder main reinforcement. The device comprises a panel (1), a positioning cone (2), a wire box (3) and a measuring wire (4); the panel(1) is of a fan-shaped frame structure, the positioning cone (2) is arranged at the bottom surface of one end of the panel (1), and the tip end of the positioning cone (2) is arranged downwards and iscoaxial with the central axis of a cylinder; the wire box (3) is arranged on the top surface of one end of the panel (1), and the measuring wire (4) is arranged in the wire box (3) and is led out through a wire outlet in the wire box (3); the center of the top surface of the positioning cone (2), the wire outlet of the wire box (3) and the circle center of the panel (1) coincide; angle scales (11) are arranged at the other end of the panel (1), and length scales are arranged on the measuring wire (4). The cylinder main reinforcement can be accurately positioned, so that the construction process is simplified, and the construction precision is improved.