Negative liquid crystals is an effective way to increase the transmittance and contrast ratio for the high-resolution small size fringe field switching (FFS) mode display. Bright line comes up on dark state at the edge of display monitor by using negative liquid crystals, which is called line Mura. The line Mura is analyzed by electric and optical simulation and experiments. Dummy pixel around the display monitor is found to be the direct cause, which leads to anomalous electric field in crab region to switch liquid crystal on dark state. The influence factors of line Mura are analyzed, and improvement method is proposed, which provides good guidance for the design of negative LCD in FFS mode in the future.