The invention discloses a flying shear locking device and a mounting detection method thereof and belongs to the technical field of flying shears of rolling mills. According to the flying shear locking device, a locking hydraulic cylinder is arranged on a flying shear holder; a regulating device is arranged between the locking hydraulic cylinder and the flying shear holder; one end of a connecting device is connected with the locking hydraulic cylinder; one end of a locking hook is connected with the other end of the connecting device; a first measurement block is movably connected with the other end of the locking hook; a second measurement block is movably connected with the other end of the locking hook; and the thickness of the second measurement block is larger than that of the first measurement block. By means of the flying shear locking device and the mounting detection method thereof, the effects that the locking force of the locking cylinder is sufficient and uniform in the mounting process of the flying shear locking device, and the extension lengths are coincident during releasing are ensured, and the flying shear is stable and reliable after being assembled to the line and applied.