Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degradation of beam quality and currently limits the power scaling of diffraction-limited high power fiber laser. The investigation progress on MI in high power fiber amplifiers is introduced in detail. The characteristics of MI are described and the ways to mitigateMI and increase the threshold power are summarized. The development trends of study on MI in high power fiber amplifier are briefly discussed.