The fault current of the inverter is limited to two times of the rated current, which causes the huge difference of the fault current in grid-connected and islanded mode of micro-grid containing inverter type micro source. The conventional overcurrent protection based on the fixed value is not suitable for the micro network any more. There are many factors that cause the uncertainty of fault current, which are summarized, such as, the operation mode of micro-grid,the exit,the capacity ,the layout and the control mode of DG, and analyzed their impacts. Current domestic and international research hotspot of microgrid line protection are summarized, and also maked classifications on whether rely on communications and realization method of protection principle. Finally, it is discussed the possible future development direction of the microgrid line protection.