Abstract: A porous magnetic filter having a sensor includes a casing having a hollow tube, a top cover located at one end of the hollow tube, a connecting section on the top cover, a plurality of channels located on the top cover and communicating with the hollow tube, and a bottom cover movably connecting to the hollow tube; a filtering unit located in the hollow tube and having a filter paper carrier and a porous magnetic membrane enclosing the filter paper carrier; and a sensor connecting to the bottom cover. The top cover is used to connect to a variety of mechanical engines. The filter paper carrier and the porous magnetic membrane are used for double filtration and protection of oil filtration. The sensor is used to detect the use situation of the filtering unit in order to alert the user about the replacement of the filter paper carrier.