While hydrogels with both high mechanical properties and thermal conductivity hold great promise for applications in artificial cartilages, actuators, antipyretic pastes and so on, it is still challenging to fabricate such hydrogels. Here, we report a class of mechanically robust, ultrastretchable and thermal-conducting composite hydrogels by in situ polymerizing acrylamide monomers in the presence of vinyl-functionalized boron nitride nanosheets (v-BNNS). The v-BNNS bear many vinyl groups, thereby chemically crosslinking the polyacrylamide chains during the polymerization process, which not only leads to a more impact network, but also promotes the load transfer between the nanoparticles and polymer chains. As a result, the composite hydrogels with 4 wt% v-BNNS show about 700% and 680% increases in compressive stress and tensile strength, respectively, and simultaneously possess ultrastretchability of 2000%. Meanwhile, the composite hydrogels are highly anti-fatigue, as dynamic loading-unloading tests show that they have outstanding load-bearing and shape recovery properties even after 820 cycles. Moreover, due to the high thermal conductivity of v-BNNS and the covalent interaction facilitating phonon transfer between v-BNNS and matrix, a 63% improvement in thermal conductivity of the composite hydrogels can be achieved by adding only 4 wt% of v-BNNS. We further demonstrate that the composite hydrogels have good biocompatibility, and thus can be applied as antipyretic pastes with excellent ability to relieve fever and potentially as artificial cartilages due to their anti-fatigue performance.