Eight kinds of 7050 aluminum alloys with different Si contents have been prepared by cast. The influence of Si contents from 0.033% to 0.491% on microstructures and mechanical properties has been investigated. The results show that significant coarse Si-rich impurity phase has been found in as-cast microstructures when Si content is increased to 0.134%, and the content of this phase is increased with Si content increasing. The coarse Sirich impurity phases are insoluble during homogenizing annealing, hot extrusion and solution treatment, and remain in aged microstructures of alloys. The aged alloy with 0.134%Si has the highest tensile strength and plasticity at room temperature. The strength, plasticity and fracture toughness reduce significantly when Si content is higher than 0.344%. Si content should be controlled in the range of 0.121% to 0.134% to get the best strength and plasticity. To obtain a better fracture toughness, the Si content in the alloy should be further reduced.