Mounted with helical magnets, the magnetic lead screw (MLS) could realize contactless conversion between rotation and linear motion. Since the magnetic field in the MLS is essentially three dimensional (3-D), previous 2-D analytical magnetic field calculation methods fail to behave well in the accuracy, while 3-D numerical methods suffer from poorer efficiency in computation time. In this article, an analytical model is developed to predict the 3-D magnetic field distribution and the electromagnetic performance of the MLS. Both radial and parallel magnetizations of the helical magnets are considered. It is shown that the air-gap magnetic field distribution is closely dependent on the magnetization pattern of helical magnets. Both 3-D finite-element (FE) simulation and prototype experiment are performed to validate the accuracy of the proposed analytical model. The analytical results are in good agreement with those by FE analysis and experiment, respectively. And it is highlighted that the magnets magnetized in parallel result in the deformation of thrust force and torque characteristics. Finally, the impact of the number of parallel magnetized magnets on the distortion of MLS performance is discussed. The work can be served as a valuable tool for precise and fast optimization design of the MLS.