The crystal structure and piezoelectric properties of wurtzite AlN doped with different contents of Er were studied by using the firstprinciples density functional theory in this paper.The calculation results show that ErxAl1-xN crystal cell parameters,crystal cell volume and bond length of the ErxAl1-xN crystalin crease significantly with the increase of the number of doped Er atoms from 0 to 25%,and the piezoelectric properties are also improved.When x = 25%, the piezoelectric constant d33 of ErxAl1xN system is 8.67 pC/N,which is 79.5% higher than that of the pure AlN.The Erdoped AlN provide us an alternative material for piezoelectric thin film in the future.