A method of preparing a lithium phosphate-based solid electrolyte according to an embodiment of the present invention may include preparing a precursor solution which includes a lithium compound, a phosphate compound, and an aluminum compound, forming a first intermediate by performing a hydrothermal reaction process on the precursor solution, forming a second intermediate by calcinating the first intermediate, and crystallizing the second intermediate. The precursor solution may further include a metal compound or a metalloid compound. The lithium phosphate-based solid electrolyte of the present invention may have high ionic conductivity and high purity.