Negative linear compressibility (NLC) describes a unique property of certain materials that expand in one direction when exposed to external pressure. Solid electrolytes with this counter-intuitive effect offer a promising opportunity to enhance the lithium-ion conductivity, as the directional expansion can facilitate the creation of improved ion transport pathways. Here, we reported a high-pressure-induced phase transition in Li4SiW12O40 from the Keggin to the bronze structure. This high-pressure phase remained stable under ambient conditions after decompression, exhibited the unconventional mechanical behavior of NLC with a compression coefficient Kc = -9 TPa[-1]. This transition enhanced lithium-ion conductivity by more than an order of magnitude, from 0.018 mS cm[-1] to 0.22 mS cm[-1]. The NLC effect synergistically expanded the migration channels and shortened the Li[+]-Li[+] distance from 5.92 Å to 3.34 Å, collectively lowering the migration barrier. Concurrently, the NLC-induced formation of highly distorted Li[+] sites within SiO5 polyhedra weakened the binding energy between Li[+] and the framework. This promoted the formation of new bonds, which shortened interatomic distances and concentrated electron density. The resultant electronic redistribution contracted the transition-state volume, thereby generating a negative activation volume, which indicated higher carrier mobility. These findings provide new approaches for enhancing the conductivity of lithium-ion electrolytes.