Sensitive　quantification　of　protein　biomarkers　is　highly　desired　for　clinical　diagnosis　and　treatment.　Yet,　unlike　DNA/RNA　which　can　be　greatly　amplified　by　PCR/RT-PCR,　the　amplification　and　detection　of　trace　amount　of　proteins　remain　a　great　challenge.　Here,　we　combined　allosteric　probe　(AP)　with　magnetic　bead　(MB)　for　assembling　an　on-bead　DNA　synthesis　system　(named　as　APMB)　to　amplify　protein　signals.　The　AP　is　designed　and　conjugated　onto　the　MB,　enabling　the　protein　biomarker　to　be　separated　and　enriched.　Once　recognizing　the　biomarker,　the　AP　alters　its　conformation　to　initiate　DNA　synthesis　on　beads　for　primary　signal　amplification.　During　the　DNA　synthesis,　biotin-dATPs　are　incorporated　into　the　newly　synthesized　DNA　strands.　Then,　the　biotin-labeled　DNA　specifically　captures　streptavidin　(STR)-conjugated　horseradish　peroxidase　(HRP),　which　is　used　to　catalyze　a　colorimetric　reaction　for　secondary　signal　amplification.　By　using　carcinoembryonic　antigen　(CEA)　as　a　protein　model,　the　APMB　can　quantify　protein　biomarkers　of　as　low　as　0.01　ng/mL.　The　response　values　measured　by　APMB　are　linearly　related　to　the　protein　concentrations　in　the　range　0.05　to　20　ng/mL.　Clinical　examination　demonstrated　good　practicability　of　the　APMB　in　quantifying　serum　protein　biomarker.　The　on-bead　DNA　synthesis　could　be　exploited　to　improve　protein　signal　amplification,　thus　facilitating　protein　biomarker　detection　of　low　abundance　for　early　diagnosis.