The present invention provides a light reaction apparatus, comprising a culture pool with an open top. A circulation path is formed in the culture pool for circulation flow of a culture solution. A part of the circulation path is composed by at least one fluid channel. Each fluid channel is composed of a bottom wall, and two opposite side walls connected with the bottom wall, wherein at least one side wall is made of a transparent or translucent material. When the present invention is used for culturing photosynthetic microorganisms, the whole temperature of the culture solution is not high, the culture will not die due to high temperature, the light-receiving area of the culture solution is increased and the yield is raised.