The invention discloses an all solid state lithium battery cathode interface modification method. An interface modification layer is 10 nm to 50 [mu]m in thickness and composed of 1-butyl-2, 3-dimethylimidazole tetrafluoro borate, lithium salt, and additives in a certain ratio. The modification layer is prepared via the following steps: a step of weighing the 1-butyl-2, 3-dimethylimidazole tetrafluoro borate, the lithium salt and the additives in a mass ratio of (69.9-90):(5-30):(0.1-5) and heating and stirring the same evenly; a step of coating a solid electrolyte layer and/or a cathode withan obtained solution and obtaining the modification layer after standing and solidifying operation. In the present invention, the 1-butyl 2, 3 dimethylimidazole tetrafluoro borate is high in ionic conductivity and wide in electrochemical window. Heating and melting the lithium salt and the additives can help further improve electrochemical performance thereof. Using the product as an interface layer can help prevent direct contact between a lithium cathode and a solid electrolyte so as to improve electrochemical performance of a solid battery.