Analysis of trace low molecular weight (LMW) proteins in serum using the label-free imaging ellipsometry (IE) immunosensor is still a challenge due to the lack of an effective signal amplification strategy and the serious nonspecific adsorption. Herein we have developed a sandwich strategy-mediated IE (SSIE) immunosensor to enable the immunodetection of LMW protein biomarkers in serum samples. We have first found that the weak binding affinity and the insufficient surface amount density of the ligand are two important factors which hinder the detection of LMW proteins in serum using the IE immunosensor. Then we have deduced that the sandwich strategy can amplify the detection signal of IE and avoid the nonspecific adsorption in serum. As a validation of the serological detection of LMW proteins, the SSIE immunosensor has been used to accomplish the quantitative detection of procalcitonin (PCT) in serum. Compared with other PCT analysis methodologies, the SSIE immunosensor enjoys the advantages of simplicity, rapidity, and sufficient sensitivity. Furthermore, we have proposed the criteria to predict the ability of the SSIE immunosensor for the detection of LMW protein biomarkers in serum, which can make the detection of LMW proteins smart and efficient.