The colorimetric immunoassay based on gold nanoparticles (AuNPs) can hardly enable simultaneous detection of multiple biomarkers in vastly different concentrations (e.g., pg/mL-μg/mL) because of its narrow dynamic range. In this work, we demonstrate an immunoassay with tunable detection range by using peptide-mediated controlled aggregation of surface modification-free AuNPs. Alkaline phosphatase (ALP) removes the phosphate group of the peptide to yield a positively charged product, which triggers the aggregation of negatively charged AuNPs and the color change of the AuNPs solution from red to blue with naked-eye readout. We design and screen 20 kinds of phosphorylated peptides to obtain a broad and controllable detection range for ALP sensing and apply them for detecting multiple inflammatory biomarkers in clinical samples. Our assay realizes straightforward, multiplexed, and simultaneous detection of multiple clinical biomarkers with tunable detection range (from pg/mL to μg/mL) in the same run and holds great potential for chemical/biochemical analysis.