Alumina content in Nanjing Steel blast furnace slag is up to 19%, which results in worse slag fluidity, higher furnace operating temperature and higher silicon content of hot metal. Therefore blast furnace strengthening is reduced. Aiming at this problem, an by laboratory experiments the viscosity of simulating Nanjing Steel BF slag samples are measured after adding MnO, which are made up of analytically pure chemical reagents. And according to the experimental results, the influence of MnO on the properties of high alumina content BF slag is analyzed and the methods of reducing silicon content of hot metal in Nanjing Steel is educed. The results showed that the addition of MnO in high Al2O3 content slag has stronger effects on viscosity decreasing, and the decreasing of slag viscosity would make it possible to reduce the operating temperature of the blast furnace, which would result in decreasing of silicon content of hot metal.