The invention belongs to the field of all-solid-state lithium batteries, and discloses a lithium lanthanum zirconium oxide solid electrolyte. The lithium lanthanum zirconium oxide solid electrolyte comprises 60-75wt% of a polymer, 8-15wt% of a lithium salt and 15-30wt% of a lithium lanthanum zirconium oxide three-dimensional porous inorganic network, wherein the polymer is compounded in the lithium lanthanum zirconium oxide three-dimensional porous inorganic network in situ. The invention also discloses a preparation method of the lithium lanthanum zirconium oxide solid electrolyte and application of the lithium lanthanum zirconium oxide solid electrolyte in the field of lithium ion batteries. The lithium lanthanum zirconium oxide three-dimensional porous network provides a continuous lithium ion transmission channel, so that the ionic conductivity is higher. Meanwhile, due to the existence of the lithium lanthanum zirconium oxide three-dimensional porous network, certain mechanical properties are provided for the composite solid electrolyte, the growth of lithium dendrites can be inhibited, and the high-temperature performance and safety of the battery are improved. Therefore, theinterfacial compatibility and stability between the solid electrolyte and the electrode are optimized and improved, and the formed all-solid-state lithium battery has the advantages of being stable in cycle performance, high in rate capability, low in interfacial impedance and good in stability.