The possibilities of porosity adjustment in alloys of the Mg-Al-Zn system obtained by melting under a layer of flux were studied. The elements that significantly increase corrosion resistance and heat resistance, and improve mechanical strength and technological characteristics were chosen as doping components. Measurements showed that the range of porosity varied between 5.9 and 14.8%, and the relationship between porosity and strength of alloys was defined for the first time for this range.For an alloy with porosity of 14.8%, the percentage of open pores was 12.8% while the percentage of closed pores was 2%. Micro-hardness of alloys with the given porosity was 661 MPa after casting, 876 MPa after homogenizing annealing and 897 MPa after artificial aging. The tensile strength was 235 MPa. Analysis identified that the main cause of porosity was catching atoms of hydrogen from atmosphere by molten alloys during melting, casting and liquation. In order to reduce the percentage of porosity alloys were doped by metal manganese, liquid metal was processed by calcium and hexachloroethane, and casting form was treated by boron nitride. These manipulations resulted in reduction of samples porosity up to 5.9%, increase of tensile strength up to 240 MPa. Open porosity was 4.5%, while closed porosity was 1.4%. At the same time micro-hardness of cast samples was 867 MPa, 903 MPa after homogenization annealing and 961 MPa after artificial aging. Further reduction of porosity and increasing of magnesium alloys strength is possible with the use of inert gases or vacuum melting. Samples porosity can be increased by more than 14.8% with the help of melting in the hydrogen containing atmosphere.