A preparation method of lithium hydroxide includes the following steps: A. coprecipitating a lithium extraction mother solution of salt lake brine with an aluminum salt solution and a sodium hydroxide solution, aging and then performing solid-liquid separation, washing and drying to obtain lithium aluminum hydrotalcite; B. acidifying the lithium aluminum hydrotalcite to obtain a lithium aluminate solution; C. performing nanofiltration on the lithium aluminate solution for lithium-aluminum separation, and sequentially performing primary concentration by reverse osmosis to obtain a primary concentrated lithium-rich solution; D. deeply removing aluminum from the lithium-rich solution to obtain an aluminum-removed lithium-rich solution; E. performing bipolar membrane electrodialysis on the aluminum-removed lithium-rich solution to obtain a secondary concentrated lithium-rich solution; F. evaporating the secondary concentrated lithium-rich solution for concentration to obtain lithium hydroxide.