The invention discloses an oil filter for a diesel engine. The oil filter comprises a shell, a filter element and a prefiltration unit, wherein the filter element and the prefiltration unit are both arranged in the shell; the prefiltration unit comprises an outer pipe and an inner filter pipe of which the pipe wall is provided with a multi-hole filter structure; the upper pipe opening of the outer pipe is sealed; the lower pipe opening is connected with the inlet of the filter element; the inner filter pipe is arranged in the outer pipe; and the upper pipe opening of the inner filter pipe is an oil inlet; and the lower pipe opening is connected with the upper end of the inner filter pipe to form circulation. According to the oil filter disclosed by the invention, during filtering, the filtering effect is good, and clogging is not caused easily.