The invention relates to an all plastic modular photobioreactor system and a method for cultivating microalgae by utilizing the same. The reactor tank body consists of all plastic closed fasteners, a sealing element, a supporting tube and a cylindrical plastic film which are subjected to modular design. The plastic film is fixed by the upper and lower closed fasteners in an octagonal buckling mode to form the closed reactor tank body. A plurality of reactor tank bodies are combined randomly into the photobioreactor system through gas supply pipelines, water inlet and outlet pipelines, an artificial light source and other auxiliary facilities so as to realize all-weather high-density cultivation of the microalgae. A device and the method provided by the invention can be widely used for microalgae cultivation in laboratories and in the production.