Abstract: China’s coal-fired power plants emit over 5.7 billion tons of CO₂ annually. To realize the dual carbon goals in time, it is necessary to reduce the carbon emissions of coal-fired power plants. Microalgae carbon fixation can efficiently absorb CO₂ through photosynthesis and convert it into biomass, which is a highly promising technology for carbon reduction in coal-fired power plants. Light guide tubes can flexibly change the light distribution in a photobioreactor, while optical fibers can transmit light centrally with low light loss. Therefore, a microalgae photobioreactor with an optical fiber/light guide tube is proposed to expand the light-receiving area of the microalgae suspension, increase the light-harvesting of microalgal cells, and promote carbon fixation via microalgae photosynthesis. The optical simulation software was used to trace the propagation of light in the optical fiber/light guide tube, and the light intensity distribution of the tube wall was obtained. Microalgae cultivation experiments were conducted in various photobioreactors and under different input light energy conditions. The change trends of biomass yield, carbon sequestration rate, and chlorophyll content were obtained, and the impact of built-in optical fiber /light guide tubes on carbon fixation via microalgae was investigated. Results indicated that the light intensity emitted from the optical fiber terminal decreased precipitously from 30 mm to 140 mm along the side of the light guide tube. Additionally, the tube exhibited a concentrated light-emitting range. Adding a conical reflector to reflect the transmitted light at the tube bottom and designing a stepped optical fiber to emit input light from different steps can optimize the light-emitting effect of the light guide tube and make the light distribution in the microalgae photobioreactor more uniform, so that the microalgal cells far away from the light source area can receive the light energy to photosynthesize and fix carbon. At a light input of 3.3 W/L, the minimum light intensity on the side surface of the light guide comprising a two-stage stepped optical fiber and conical reflector was 47 μmol/(m²·s). The average light intensity was 64 μmol/(m²·s), representing a 2.6-fold increase compared to the light guide tube lacking both the optical fiber and any enhancements beyond its top section. After seven days of cultivation, the microalgae concentration in the stepped fiber photobioreactor (SF-PBR) could reach 1.9 g/L, which was 46.2% higher than that of the photobioreactor inserted with flat-end fiber optic light guide (FF-PBR) and 111.1% higher than that of the photobioreactor with only top-fed light (LG-PBR). When the light input was escalated to 5.0 W/L, a high microalgae concentration of 2.8 g/L was achieved at 7 d. Meanwhile, the average carbon sequestration rate of 608.3 mg/(L·d) was obtained, exhibiting 1.9-fold augmentation compared to the control LG-PBR.