A photocatalytic reaction system by collecting sunlight, the system including: a light collector, a light conduction device, and a photoreactor. A transparent protective cover is disposed on the top of a housing of the light collector. A light-collecting convex lens group is disposed beneath the protective cover in the transmission direction of the sunlight. The housing of the light collector is provided with a solar radiation measuring device. An azimuthal main shaft and the pitch main shaft are separately provided with the servo motors and are rotatable in relation to each other by tracking the sunlight under the drive of the separate servo motors. The sunlight collected by the light-collecting convex lens group is converged into a convergent light when passing through the light conduction device and the convergent light is directed to the photoreactor. The photoreactor functions to transmit full-spectrum rays of sunlight.