In order to reduce fluorine pollution, partial fluoride could be replaced by B2O3 in mould flux. The effects of B2O3 on properties of low fluoride content mould flux for thin slab continuous casting were studied. The properties included melting temperature, viscous characteristics, fluidity, surface tension and Al2O3 inclusion adsorption. The results showed that with the increase of B2O3 content, melting temperature, viscosity, viscous activation energy and surface tension of mould flux decreased obviously, fluidity increased, and Al2O3 inclusion adsorption of mould flux changed little. The B2O3 could play effective role when its content was less than 6%. The results would play a guiding role in designing of low fluoride content mould flux for thin slab continuous casting.