Polyimide (PI) is an important engineering material, but its poor thermal conductivity limits its application in the field of electronic packaging. In this work, we prepared boron nitride nanosheets (BNNSs) and boron nitride nanosheets/silver nanoparticles (BNNSs/AgNPs) hybrid fillers which combined successfully with the PI matrix by in-situ polymerization to produce high-thermal-conductivity PI composites. The results showed that when the filler addition was 20[Formula: see text]wt.%, the composites had the best overall performance, and the thermal conductivities of the BNNSs/PI and BNNSs/AgNPs/PI composite films were 0.863[Formula: see text]W[Formula: see text]m[Formula: see text] and 1.175[Formula: see text]W[Formula: see text][Formula: see text][Formula: see text]m[Formula: see text][Formula: see text][Formula: see text][Formula: see text]K[Formula: see text], respectively, which improved 506% and 726%, respectively, compared with pure PI. Furthermore, the volume resistivity of the composites is higher than 10[Formula: see text], indicating an excellent insulating property. At the same time, when used as the thermal interface material (TIM) for light-emitting diode (LED) chips, the composites have significantly improved heat dissipation effect and excellent mechanical properties at 75∘C, which are expected to be widely used in the field of electronic packaging.