For lab-scale application, both the top-irradiated cylindrical and side-irradiated irregular reactors have been employed. They share similarities in their small scale, high mass transfer rate, and the existence of free liquid surface led by magnetic stirring. However, in the top-irradiated cylindrical reactor, incident radiation usually has to pass the gaseous region and will be partially absorbed by the water spray. In such a top-irradiated reactor, enough negative pressure has to be kept to avoid droplet formation. In a side-irradiated irregular reactor, the catalyst-liquid suspension is directly illuminated; thus, it can be operated under a normal pressure. The aforementioned magnetically stirred photocatalytic reactor (MSPR) is a typical side-irradiated irregular reactor. Despite the existence of the planar light-receiving window, the curved boundary of the reactor as shown in Fig. 1 enables the growth of flow loops without causing unnecessary vortex. Owing to its convenience and economic applicability, MSPR has been employed in numerous lab-scale experimental studies and achieves an acknowledged performance.1,3,20,21 In our lab, in particular, MSPR has been integrated with an automatic sampling system for long-term hydrogen generation evaluation.1 However, the optimization work of such photoreactor is rare. It apparently needs parameters such as the reactor volume, optimal photocatalyst loading, and stirring speed. These can be achieved by numerical simulation methods, which can significantly reduce the laborious experimental work, broaden the range of experimental investigation, and provide some valuable theoretical guidance. In this paper, the flow field with free liquid surface, photocatalyst and photonic flux distributions in MSPR will be proposed comprehensively. It is expected that this work will potentially be an initial step for this lab-scale photocatalytic reactors’ performance optimization.