As the most suitable thermal energy storage manner for the Joule-Brayton based Pumped Thermal Electricity Storage (PTES), packed beds thermal energy storage has the natural feature that a steep thermal front propagates with great difference of temperature and density, which lead to an unbalanced mass flow rate of packed bed reservoirs and the PTES close loop. In this paper, the expression of thermal front propagation and unbalanced mass flow rates between the inflow and the outflow of packed beds is developed and validated experimentally. The result indicates that there are 0.62%, 0.26% and 0.36% unbalanced mass flow for the hot reservoir, the cold reservoir and the PTES close loop respectively. The sensitivities of the factors such as pressure ratio, heat capacity of TES material and porosity on the unbalanced mass flow rate and the round-trip efficiency of PTES system considering mass flow rate is discussed. Furthermore, a feasible and self-balancing PTES system without buffer vessel is proposed, with a round-trip efficiency 0.12% higher than the buffer vessel balancing PTES system.