The invention provides a method for preparing surface enhanced Raman substrate, and relates to Raman spectrums. The method comprises the steps that a seed crystal growing method is used for synthetizing a gold nanorod solution, wherein gold nano seeds are synthetized first, then a growing solution is added, silver nitrate is used for adjusting the slenderness ratio of gold nanorods until reaction is completed, the needed gold nanorod solution is obtained, then centrifuging is conducted to remove supernatant liquor, signal molecules are added to be connected to a gold surface, then a polyethylene glycol dressing agent is added to induce the gold nanorods to conduct self-assembling, then a processed nano particle solution is stirred, a TEOS methanol solution is added for reacting, silicon dioxide layers are wrapped on the surfaces of the self-assembled gold nanorods, and the surface enhanced Raman substrate is obtained. Detecting can be directly conducted in the solution, due to self-assembling and the coupling effect of plasma on the surfaces of adjacent nano particles, the electromagnetic field between the nano particles is obviously enhanced, the Raman signals at the position are amplified in a quantity level mode, accordingly, the detecting sensitivity of surface enhanced Raman is greatly improved, and the stability is high.