A pump providing oil suction and oil drain functions includes a barrel and a lid; a first passage is disposed on a bottom of the barrel; the lid contains a second passage, a third passage, and a fourth passage; a regulating member is disposed on the lid to connect the third passage through the second passage and to allow air to flow in sequence from an oil storage space, the third passage, the second passage, a pipe, the first passage, a ventilation pipe, and the fourth passage into the ambient air for producing suction in oil suction operation; and to connect the fourth passage through the third passage for the ambient air to flow in sequence through the second passage, the pipe, the first passage, the ventilation pipe, the fourth passage, and the third passage to enter into the oil storage space for producing pressure during the oil drain operation.