The invention discloses a longitudinal positioning and connecting device for a support arch frame of an underground project and a construction method. The longitudinal positioning and connecting device comprises a connecting rod, a first fixing device and a second fixing device, wherein one end of the connecting rod is a spherical round head, and a first threaded hole is formed in the spherical round head; the other end of the connecting rod is of a swelling structure; the first fixing device is used for providing a clamping structure for clamping and fixing the swelling structure, and the inner diameter of the clamping structure is smaller than the outer diameter of the swelling structure; and the second fixing device is of a hollow structure formed by surrounding of a shell, the end part of the hollow structure is opened, a second threaded hole matched with the first threaded hole is formed in the shell, and the diameter of the spherical round head is larger than the inner diameter of the end part of the hollow structure and smaller than the inner diameter of a main body part of the hollow structure. The longitudinal positioning and connecting device has the beneficial effects that the positioning is convenient and flexible, and the construction period is shortened. By utilizing the device, the longitudinal connection and accurate positioning of the support arch frame can be realized by only moving the arc frame, aligning the arc frame and inserting the arc frame into the first fixing device, so that the construction time is greatly saved, and the construction safety is improved.