The main useful mineral in a magnesite ore of liaoning province is magnesite, the main gangue minerals are dolomite, pencil stone, quartz and so on. The content of MgO is 93.80%, the content of the main impurity SiO2 is 1.24%. The indexs of experiment of contents of SiO2 0.13%, content of MgO 97.17% in concentrate, yield of concentrate being 71.66% is got on the condition of fitting grind fineness and medicament by using the reverse flotation flowsheet.