Polymer nanocomposites containing high K ceramics have been developed for boosting the energy density of dielectric capacitors. However, there are numerous challenges in the research about how to optimize the electric field distribution and improve the interfacial structure of nanocomposites for overcoming dielectric mismatches between high K nanofillers and low K polymers. Herein, all-chemical bonding cross-linked nanocomposites were designed and nano-BT with different sizes were regarded as cross-linked points rather than a free dispersed phase in polymers. In addition, the cross-linking degree could be controlled by changing the nano-BT sizes. 60 nm BT-BCB@DPAES nanocomposites possess the most excellent mechanical and thermal properties as well as the highest theoretical breakdown strength. In fact, 100 nm BT-BCB@DPAES nanocomposites have the most perfect dielectric performance combined with the experimental data and finite element simulation, particularly at 150 °C, the highest breakdown strength of 442 MV m-1 and greatest discharged energy density of 3.1 J cm-3 were obtained. This is attributed to the proper cross-linking degree and uniform electric field distribution. Overall, this kind of cross-linked structure can effectively enhance dielectric performance, particularly at elevated temperatures. This provides an idea for developing high temperature polymer nanocomposites for dielectric energy storage applications.