Flexible energy storage devices that can function under considerable physical deformation have shown great promise for applications in portable electronics. In this work, we report for the first time a novel design of all-solid-state symmetric supercapacitors (SCs) based on free-standing ultrathin boron-doped graphene paper (B-GP) electrodes with boron-enriched gel polymer electrolytes (GPEs). Specifically, the B-GP electrodes integrated with B-containing GPEs not only introduce more electrochemically active sites for absorption/desorption of electrolyte ions, but also facilitate diffusion of the electrolyte during charging/discharging processes. The novel design of B-GP with B-containing GPEs endows our symmetric SC with a voltage of 2.5 V, an unprecedented energy density of 39.31 W h kg⁻¹ (with a power density of 1.44 kW kg⁻¹), a stable cycling performance (a capacity retention of 90% after 3000 continuous charge/discharge cycles), good rate capability, distinguished mechanical flexibility and temperature resistant stability. This work provides a promising candidate to design a new generation of electrochemical SCs for energy storage devices.