The invention discloses an inorganic/organic nanometer composite solid electrolyte and a preparation method thereof, belonging to the inorganic/organic nanometer composite material and the preparation process field. The solid electrolyte is composed by polyethylene oxides PEO, lithium perchlorates LiClO4 and hydrotalcite nanometer sheets LDHNS, has a chemical formula of PEO/LiClO4/LDHNS, the massparts of each component are 54-91%, 8-16% and 1-30% respectively. The preparation method of the composite solid electrolyte comprises: mixing PEO, LiClO4 and LDHNS in water equably and sufficiently, and then vaporizing the solvent to obtain the composite solid electrolyte of the invention. The inorganic/organic nanometer composite solid electrolyte according to the invention has high conductivity, high lithium ion transference number and heat stability, the electrolyte can be used as the electrolyte of a solid lithium ion battery.