A breast pump includes a main body, a breast milk suctioning shield, and one or more air pumps. The breast milk suctioning shield is assembled in an accommodation space of the main body and detachably connected to the main body. A front end of the breast milk suctioning shield has a breast shielding portion, and a nipple passage extends from a rear end of a center portion of the breast shielding portion. One or more deformable members and a non-deformable supporting member are assembled with an annular connection portion located between the breast shielding portion and the nipple passage. The air pump has an intake port and an exhaust port. The intake port is controlled by a first valve controller to suction the nipple passage and generate a negative pressure, and the exhaust port is controlled by a second valve controller to inflate or deflate the deformable member.