The present disclosure relates to a modularized photosynthetic reactor system and an algae culturing method using the same, in which the modularized photosynthetic reactor system includes at least one reactor, an air pipe, an out-feed tube, an in-feed tube and an exhausting pipe. Moreover, each reactor is composed of a plurality of clamp sets and at least one membrane cartridge, and the algae culturing method comprises: a sterilizing procedure, a culturing procedure and a harvesting procedure. In the sterilizing procedure, water, air and a hypochlorite solution are fed into a reactor for sterilization and then a sodium thiosulfate solution is fed into the same reactor for chlorine neutralization. In the culturing procedure, a culture medium and an algae liquid are fed into the reactor for allowing algae to grow until a steady growth period is reached. In the harvesting procedure, the algae grown in the reactor is harvested after the steady growth period is ended.