The invention provides a synthesis method and a new application of potassium octafluoro hafnate. The synthesis method comprises the following steps of: step 1, preparing a transparent H4HfF8 solution; step 2, preparing a 20-30% lithium fluoride suspension from lithium fluoride by pure water; step 3, dripping the lithium fluoride suspension prepared in the step 2 in the transparent H4HfF8 solution prepared in the step 1, and preparing potassium octafluoro hafnate H4HfF8 suspension; and step 4, filtering the potassium octafluoro hafnate H4HfF8 suspension prepared in the step 3, washing the filter cake with pure water, crystallizing, then placing the crystal on a PTFE (polytetrafluoroethylene) plate, heating, drying, naturally cooling, then grinding and crushing, so as to obtain otassium octafluoro hafnate powder. The synthesised otassium octafluoro hafnate is high in purity, and capable of obviously enhancing the electrical conductivity of a lithium battery electrolyte in case of being applied to the lithium battery electrolyte containing lithium hexafluorophosphate, thus improving the performance of a lithium battery.