This study aims to elucidate the mechanism of platelet thrombus growth from a rheological perspective. We performed in vitro experiments by perfusing porcine blood through a microfluidic channel with a collagen-coated constricted section. The detailed behavior of platelet aggregation was observed using a laser confocal microscope on fluorescently labelled platelets, and a three-dimensional model of the platelet aggregates was constructed. By combining this with computational fluid dynamics, we calculated the shear rate on the surface of the platelet aggregates and the growth rate of the aggregates, and examined their relationship. The results of comparing the growth rate of platelet aggregation when the initial shear rate is changed by varying the perfusion flow rate showed that the growth rate becomes almost constant with increasing shear rate when the initial shear rate is high, but increases with increasing shear rate when the initial shear rate is low. Furthermore, when the initial shear rate is high, platelet aggregation grows uniformly in all directions, but when the initial shear rate is low, growth is less in the left and right directions of flow and more downstream. This is due to the fact that at excessively high shear rates, the growth of platelet aggregation is reduced, and where there is a gradient in the reduction of shear rate relative to flow, platelets tend to aggregate more readily. These findings contribute to the basic understanding of platelet thrombus formation and are expected to be a step toward applications in medical devices where thrombosis is an issue and in thrombosis testing devices.