The invention relates to an oil pump and a method for lubricating a chain of a gear chamber. The oil pump comprises an oil pump shell and an oil spray hole, wherein the oil spray hole is formed on the oil pump shell and used for spraying oil outwards. In the lubricating method, the oil pump is adopted to lubricate the chain. When an engine is started, oil is sprayed onto the chain along the direction of the oil spray hole so as to lubricate and cool the chain.