This paper introduces a new portable platelet aggregation instrument based on optical transmittance measurement. The platelet-rich plasma is aggregated after adding trapping agent and leads to a transmittance enhancement. By analyzing the transmittance evolution curve, the platelet aggregation function can be obtained, which is helpful for doctors to do clinical diagnosis of thrombotic disease and make a prescription of drugs. The design of the optical module, the signal processing circuit, and the software are described in detail. After assembling the instrument, several tests of blood samples are carried out to verify its performance. The instrument is compact and easy to use, so it is potential for rapid detection or real-time monitoring.