The invention discloses a lubricating oil cooler for a marine main engine. The lubricating oil cooler comprises a main shell. A front shell and a rear shell are separately arranged on the front side and the rear side of the main shell. A first groove is formed in the connection face of the front shell and the main shell. A first filter screen is arranged in the first groove. A second groove is formed in the connection face of the rear shell and the main shell. A second filter screen is arranged in the second groove. An oil inlet is formed in the upper end of the front shell. An oil outlet is formed in the upper end of the rear shell. A water inlet pipe and a water outlet pipe are arranged at the upper end of the main shell. Cold water is injected into a circulating water pipe through the water inlet pipe. Lubricating oil is guided into the front shell through the oil inlet, large granular mechanical impurities in the lubricating oil are filtered out through the first filter screen, andthen the lubricating oil flows into the main shell; cold water is injected into the circulating water pipe and exchanges heat with the lubricating oil, so that the lubricating oil is cooled; and small granular mechanical impurities in the lubricating oil are filtered out through the second filter screen.