Abstract: This paper studies the existent form of yttrium in Al-Cu-Mg-Mn alloy and the effects of yttrium 0 %～ 0.4 %，mass fraction on microstructure and properties of Al-Cu-Mg-Mn alloy using optical microscope，scanning electron microscope and X-ray diffraction. The results showed that the cast microstructure of Al-Cu-Mg-Mn alloy was gradually fined when quantity of yttrium was under 0.3 % (mass fraction). The grain size dropped and homogenization was fined remarkably with addition of 0.3 %yttrium．The cast microstructures of the alloy gradually become coarse when the percentage of the yttrium increased to more than 0.3 %. The Cu4Al8Y resulting from addition of yttrium to the Al-Cu-Mg-Mn alloy was distributed at the grain boundary, which is beneficial to improve the property of aluminum alloy.