The influence of complex Cr, Yb and Zr alloying on the microstructures and fracture characteristics of Al-Zn-Mg-Cu alloy has been investigated and the mechanism of toughening by adding Cr, Yb and Zr was analyzed. The results show that the Al-Zn-Mg-Cu alloy containing Cr, Yb and Zr may form the spherical dispersoids containing the Cr, Yb and Zr homogeneously distributed in the Al matrix, which can strongly pin the dislocation and subgrain boundaries, retain the deformation-recovery Al matrix with low angle boundary in the alloy and inhibit the recrystallization of Al matrix. By complex Cr, Yb and Zr alloying to Al-Zn-Mg-Cu alloy tempered as T6, the fractographs consist of dimple-type transgranular cracking with minor intergranular cracking. Compared with the Al-Zn-Mg-Cu-Zr alloy, the intergranular fracture resistance increases obviously, the precipitates free zone is wider with discontinuous distribution on the grain boundaries for the precipitates