The invention relates to a transmission mechanism of a diesel engine overhead camshaft, including a gear room installed on a cylinder block of the diesel engine, the camshaft and a crankshaft, wherein, a chain transmission device and a wheel gearing are arranged in the gear room from top to bottom; the crankshaft, the wheel gearing, the chain transmission device and the camshaft are connected in a transmission way sequentially. The transmission mechanism of the invention realizes the overhead camshaft structure on the traditional diesel engine on which the camshaft is arranged laterally, which improves the high speed performance of the diesel engine.