Abstract: By using a cold CFB model, the effect of horizontal orifice areas of Loop Seal with side aeration air on operational property was studied in detail.The results showed that the horizontal orifice area had a great effect on its stable operation and regulation of the recirculation rate.Under a certain range of the horizontal orifice area,the stable flow of solid particles through Loop Seal could be obtained and recirculation rate of solid particles could be well regulated and controlled through changing external aeration.The flow resistance properties across the horizontal orifice were given and an empirical equation of the pressure drop of horizontal orifice and solid circulating flow rate was proposed and a new orifice discharge coefficient was defined.The expressions were in good agreement with the experimental data.