<p>To minimize the high energy cost of biomass production in the large-scale production of <i>Phaeodactylum tricornutum</i>, the diatom was cultured outdoor under solar light. The damage caused by outdoor high-light stress to the photosynthetic apparatus is a major threat to the productivity of <i>P. tricornutum</i>. We demonstrated that the addition of alginate-derived oligosaccharides (ADOs) could significantly enhance the high-light resistance of photosystem II (PSII) in <i>P. tricornutum</i>. The ADOs decreased PSII photoinhibition and increased the electron transport rate under high light. The beneficial impact of the ADOs in reducing light inhibition was more noticeable under increased light intensities. This positive effect was attributed to two factors. The first: the ADOs lowered the capability for absorbing light energy in the PSII reaction centers, protecting PSII from excess light energy under high light. The second: the ADOs enhanced the activity of antioxidant enzymes (APX and SOD), reducing the over-accumulation of reactive oxygen species (ROS, i.e., H₂O₂), which further decreased the ROS-induced net degradation of D1 protein, ensuring the smooth photosynthetic electron transport of PSII under high light. Given the enhancement of high-light resistance in <i>P. tricornutum</i>, ADOs could serve as a protectant, which can be applied for large-scale production of <i>P. tricornutum</i> under outdoor culture conditions.</p>