Abstract: High-Mg andesites are occurred in Zakeshan area,North Qilian, which were formed in Early and Middle Cambrian, with Rb-Sr isotopic age of (516±7)Ma (1σ).These andesites (SiO₂=51.45%~59.62%,averaging in 56.33%)have higher MgO contents (MgO contents are 4.85%~9.64%, averaging in 71.44%, Mg^# values are 66.68~74.15, averaging in 71.44)than the ones of typical island arc andesite, with enrichment of Cr (21.6×10^-6~476×10^-6, averaging in 196.26×10^-6), Ni (14×10^-6~102×10^-6, averaging in 56.14×10^-6)and Na elements, but low Al (Al₂O₃=13.22%~17.37%, averaging in 15.69%), Ca (CaO=2.01%~10.33%, averaging in 5.64%)and K (K₂O=1.04%~2.79%, Na₂O/K₂O=0.59~5.0, averaging in 2.36).Meanwhile, these rocks show weakly negative Eu anomaly, they are enriched in Rb, Th and K (LILE), but they are depleted in Sr, Ti, P, Nb and Ta (HFSE), with LREE/HREE ratios ranging from 7.19 to 13.58 (Yb=1.47×10^-6~3.26×10^-6, Y=11.6×10^-6~22.5×10^-6). Combined with regional geological setting, the results suggest that high-Mg andesite was formed in island arc setting and was sourced from the partial melting of the peridotites in mantle wedge, which were metasomated by the fluids from subducted sediments and altered oceanic crust.