The invention provides an LED (light emitting diode) preparation method. The method comprises following steps: a), an LED epitaxial slice grows with a MOCVD (metal organic chemical vapor deposition) method; b), a window layer and a transparent substrate are subjected to bonding treatment; c), the substrate is removed, and a corrosion barrier layer is removed; d), an electrode pattern is prepared on an Ohmic contact metal layer; e), the Ohmic contact metal layer is subjected to alloy treatment; f), a metal reflecting mirror is prepared; g), a negative electrode area is prepared by etching; h),an insulating protection layer is applied by coating; i), a positive electrode channel and a negative electrode channel are prepared by photoetching; j), a positive electrode and a negative electrodeare prepared by stripping; k), the transparent substrate is subjected to thinning treatment, and the edge of the transparent substrate is subjected to chamfer treatment; l), the LED epitaxial slice prepared by the steps a)-k) is cut and tested. The built-in metal reflecting mirror is prepared while the transparent substrate is replaced, so that stability is greatly improved, light extraction efficiency is improved, and light absorption caused by aging of an encapsulating material at a later stage is avoided; by means of the insulating protection layer, the same height of the positive electrodeand the negative electrode can be guaranteed, and encapsulating yield is effectively increased.