To cultivate more microalgae and at the same time optimize the land use, the optimal light exposure duration and frequency are important factors in the design a photobioreactor which contained two parts was tested. One part of the PBR was exposed to the light, while another part of the PBR was placed underground without light. Different daily exposure durations (8 hours, 7 hours, 6 hours, and 5 hours) and different exposure frequencies (53 cycles/hour, 70 cycles/hour, 88 cycles/hour, and 105 cycles/hour) were investigated. A comparison of the growth rates and biomass characteristics of microalgae under different light conditions showed that, the biomass concentration in the aqueous phase can reach up to 3.5g/L when the daily light duration was 7 hours and frequency was 70 cycles/hour. This experimental result implies that the concept of this two portions photobioreactor can work to produce microalgae biomass under certain light exposure durations and frequencies. It is a new approach to discover the light-dark cycle for microalgae growing.