The dynamic characteristics of a solar thermal storage system using a packed bed with spherical capsules during the discharging process are studied. According to the energy balance of a solar thermal storage system, the dynamic discharging processes model of a packed bed with spherical capsules is presented. Paraffin is taken as a phase change material and water is used as a heat transfer fluid. The temperatures of phase change material and heat transfer fluid, solid fraction, and heat released rate are investigated. The effects of inlet temperature of heat transfer fluid and flow rate of heat transfer fluid on the time for discharging and heat released rate are also discussed.