The invention discloses a preparation method of a quantum dot/enzyme compounded carbon paste electrode for detecting SAM (S-Adenosylmethionine). The preparation method is characterized by comprising the following steps of doping water-soluble CdZnTe quantum dots and transmethylase in a carbon paste electrode; by taking 1-ethyl-3--methylimidazolium hexafluorophosphate as an adhesive, mixing a carbon nano-tube with graphite powder to prepare the carbon electrode, so as to obtain the quantum dot/enzyme compounded carbon paste electrode; compared with an ordinary carbon paste electrode, the quantum dot/enzyme compounded carbon paste electrode has the advantages of conducting property improved by 1 to 2 times, wide electrochemical window, simple preparation method, low cost, easy update for surface, low residual current and the like; the eelctrode can be used for quickly detecting the SAM in a sample; the method has the characteristics of high sensitivity, good selectivity, short response time and low interference, is superior to other detection methods, and is a determination method with simpleness, quickness, convenience and feasibility for the SAM. A fixed methyltransferase electrode sensor prepared by the preparation method disclosed by the invention is low in cost, simple in preparation process and good in specificity, and has the possibility of realizing automated field determination.