The invention provides a connecting-rod deep hole grinding method for a marine low-speed diesel engine and a special honing device. The connecting-rod deep hole grinding method comprises the followingsteps: 1, grinding machined traces at a rounded corner of a deep hole by using a long-handle plane grinding head; 2, grinding relatively deep vibration knife marks and machined traces as well as spiral lines near an orifice of the deep hole by using a plane grinding head; 3, grinding and smoothening the entire hole wall by using the special honing device, wherein the special honing device comprises a honing head, a universal quick-change adapter and a pneumatic motor and is mounted on an extension rod, the hole wall of the deep hole is tightly supported through an elastic structure, the special honing device is driven by a pneumatic grinder to rotate, the rotating speed of the special honing device is adjusted by using a speed reducer, and then the uniform and efficient deep hole grindingoperation is achieved; 4, grinding burrs and shallow traces in the deep hole by using a sand impeller head; 5, removing waste and burrs on the hole wall by using a steel wire brush; 6, cleaning the hole wall; and 7, carrying out repairing and grinding till quality testing is passed. According to the connecting-rod deep hole grinding method provided by the invention, the grinding effect is uniform, the operation is simple, both the time and the effort are saved, and the entire grinding effect on the deep hole in a connecting rod is improved.