The borehole spacing and water pressure of water curtain system have a significant influence on water sealing effect of underground oil storage cavern. The numerical model of underground oil storage cavern is established; the influence of parallel fissure on seepage field is analyzed; the influence of borehole spacing and water pressure of water curtain system on water sealing of underground oil storage cavern under parallel fissures is studied. The study results show that: 1) The influence of water curtain system on oil storage cavern and that of parallel fissures on transmission of seepage pressure in surrounding rocks are obvious. 2) The superposition effect can be improved by reducing borehole spacing and increasing water pressure rationally. 3）The influence of water pressure on water curtain system is larger than that of borehole spacing on water curtain system; as a result, increasing water pressure is superior to reducing borehole spacing in terms of improving water sealing effect of underground oil storage cavern. 