The invention provides a method for preparing lamivudine. The method comprises the following steps of: directionally synthesizing an impurity J by using uracil as a raw material, and confirming the structure of the impurity J, namely performing high performance liquid chromatography (HPLC) test to obtain relative retention time of the impurity J; and performing quality control on synthesizing raw materials for the lamivudine, namely selecting cytosine of which the uracil content is less than 1.0 percent as a raw material and thus obtaining the lamivudine of which the impurity J content is less than 0.1 percent.