In the present article, the high signal-to-noise ratio surface enhanced Raman spectrum of rat serum was analyzed. To be high efficient substrate of SERS, the novel silver colloid was synthesized by microwave method. The characteristic peaks of the rat Raman spectrum were assigned. The mechanism of "hot spot" which was formed by aggregated silver particles was explained by electromagnetic fields enhancements theory, i. e. the excitation of plasmons in the metallic nanoparticles created greatly enhanced local electromagnetic fields, contributing to the major component of the SERS effect. The field distribution at novel silver aggregates exhibits a dramatic enhancement. These strongly localized fields provide an important mechanism for surface enhanced Raman scattering. The surface enhanced Raman spectrum of low quality of component in rat serum was acquired so that the change of body could be known in time. A new method for acquiring abundant information of macro-biomolecule by spectrum means was brought out. It is hopeful to be a novel technique to diagnose diseases in the early stage with serum.