We present an optical sensing platform on a smartphone for high-throughput screening immunoassays. For the first time, a designed microprism array is utilized to achieve a one-time screening of 64 samples. To demonstrate the capability and the reliability of this optical sensing platform on smartphone, human interleukin 6 (IL-6) protein and six types of plant viruses are immunoassayed. The ability of quantification is shown by a sigmoidal dose-response curve fitting to analyze IL-6 protein. The accuracy in measuring the concentrations of IL-6 protein achieves 99.1%. On the other hand, to validate on-field immunoassays by our device, a total of 1030 samples are assayed using three immunoassay methods to detect six types of plant viruses. The accuracy is up to 96.2-99.9%; in addition, there is a high degree of agreement with lab instruments. The total cost for this high-throughput optical screening platform is ∼$50 USD. The reading time is only 2 s for 64 samples. The size is just as big as a portable hard drive. Our optical sensing platform on the smartphone offers a route toward in situ high-throughput screening immunoassays for viruses, pathogens, biomarkers, and toxins by decentralizing laboratory tests. With this mobile point-of-care optical platform, the spread of disease can be timely stopped within a very short turnaround time.