The invention provides an underwater horizontal type diamond-wire saw. The underwater horizontal type diamond-wire saw comprises a clamping and fixing part (1), a cutting part (2), a string bead wire anti-lock device (3), a feed part (4), an underwater hydraulic control valve box and a detecting system (5), and underwater horizontal type diamond-wire saw is in a horizontal type state and clasps on a workpiece (9) to be cut when being in use. The underwater horizontal type diamond-wire saw has strong clamping force and simple operation, the complete machine has better stress state, more stable and reliable operation when completing the cutting operation. The invention can adapt to underwater standpipe operation mode and special environmental needs. The wire saw can be clamped on a major diameter pipeline, the clamping force can overcome bending moment generated by the gravity and the feeding force; meanwhile, a guide post and a guide sleeve of the radial feed are respectively arranged on a cutting frame and a clamping frame, the design manner can lead the feed force to finally act on the cut pipeline, thus improving the overall stress state of the wire saw, effectively avoiding screw deformation, ensuring the normal operation of the diamond-wire saw, and the operation is obviously improved.