In recent years, the proportion of the wind power in the grid is increasing rapidly. In addition, the penetration of wind power in grid has been rising. It will lead adverse effect to grid, if the wind turbine off-grid during the fault of network. Wind power technology-leading countries have issued the quantitative standards of fault through which requires on-line operation .In this paper, the influence of wind turbines off-grid on the grid and turbines during the fault of grid is analyzed firstly. Meanwhile, the control objective of the low voltage ride through (LVRT) is proposed. Several types of LVRT technology for the Permanent Magnetic wind Power System are summarized and evaluated respectively. Finally, the developing direction of this technology is drawn from the above discussion.