The Er-containing Al-Mg alloy plates were processed by different heating treatments. The effects of microstructure on fatigue crack propagation of the plates were analyzed. The results show that, the more slender of the grain, the faster of fatigue crack propagation rate; with the reduction of fatigue crack propagation rate, the crack width increase and crack path twists and turns. Due to the addition of rare earth element Er, formed Al3 (Er, Zr) peticipate particles in the alloy, the Al3 (Er, Zr) pin the dislocation, strongly hinder dislocation motion, in order to reduce the stress concentration in the grain boundary produced by the dislocation pile-up, and then reduce the fatigue crack growth rate.