In order to reduce fluorine pollution, partial fluoride could be replaced by K2O in mould flux. The effects of K2O on melting temperature, viscosity, fluidity, surface tension, Al2O3 inclusion and adsorption of low fluoride content mould flux for high speed continuous casting were studied. The results showed that with the increase of K2O content, melting temperature, viscosity and surface tension of mould flux decreased obviously, and fluidity and Al2O3 inclusion adsorption increased. When basicity was 0.81, 1.03 and 1.22, the suitable content of K2O was 5%, 5% and 4% respectively, which could play an effective role. Partial fluoride could be replaced by K2O in mould flux.