Abstract: Based on the effects of surface tention--capillary, the behavior of gas bubblegrowth and detachment from submerged vent ports and the fundamental theory offluid mechanics,the basic principles of double vent diving bell structure is discus-sed,the reference design is presented. Analysis indicates that the necessary condi-tion for no Na permeation from coolant to diving bell ought to be p_g--p_l>4σ/d_i,the preferable value of d_i ought to be<0.3mm, v is not a critical limited parame-ter,for under--medium burnup design, vented option has no significant advantages,the wetted and vented option is particularly preferred.