Highly sensitive, multiplexed detection of soluble cancer protein biomarkers can facilitate early cancer screening as well as enable real-time monitoring of patients' sensitivity and resistance to therapy. Current technologies for detection of soluble cancer protein biomarkers, e.g., enzyme-linked immunosorbent assay, however, suffer from limited sensitivity, as well as the requirement of expensive monoclonal antibodies, which undergo the quality variability. Herein, we propose a sensitive, cheap, and robust surface-enhanced Raman scattering technology to detect a panel of soluble cancer protein biomarkers, including soluble programmed death 1 (sPD-1), soluble programmed death-ligand 1 (sPD-L1) and soluble epithermal growth factor receptor (sEGFR), which are related to disease progression and treatment efficacy. In this assay, gold-silver alloy nanoboxes that have strong Raman signal enhancement capability were used as plasmonic nanostructures to facilitate highly sensitive detection. In addition, nanoyeast single-chain variable fragments were utilized as mAb alternatives to allow specific and stable protein capture performance. We successfully detected sPD-1, sPD-L1, and sEGFR with a limit of detection of 6.17 pg/mL, 0.68 pg/mL, and 69.86 pg/mL, respectively. We further tested the detection of these three soluble cancer protein biomarkers in human serum and achieved recovery rates between 82.99% and 101.67%. We believe our novel platform that achieves sensitive, multiplexed, and specific detection of soluble cancer protein biomarkers could greatly benefit cancer treatment and improve patient outcome.