The effects of LED monochromatic light quality of different colors on the growth of phaeodactylum tricornutum cells, their fucoxanthin content, and the expression levels of genes related to fucoxanthin biosynthesis are investigated. The results show that red light, green light, and purple light promote the growth of phaeodactylum tricornutum cells. In addition, the number of algal cells significantly increases by 12.44% on the 6th day under red light treatment (P<0.01) compared with that in the control group. Yellow light and blue light inhibit the growth of phaeodactylum tricornutum cells, and the aging rate of algal cells under green light and purple light treatments is higher than that in the control group. Compared with the control group, the fucoxanthin content of phaeodactylum tricornutum treated with red light and purple light increases by 16.61% and 26.78%, respectively, and that of phaeodactylum tricornutum treated with blue light and green light significantly reduces (P<0.01). The changes in chlorophyll and fucoxanthin content, which are related to photosynthesis, are basically the same under all treatments. Evaluation of the expression levels of genes related to fucoxanthin biosynthesis shows that among zeaxanthin cyclooxygenase gene (zep), phytoene synthase gene (psy), ξ-carotene dehydrogenase gene (zds), lycopene β-cyclase gene (lcyb), carotene isomerase gene (crtiso), and phytoene dehydrogenase gene (pds), the expression level of the key gene psy, is consistent with the change in the fucoxanthin content, whereas the expression levels of pds and lcyb are significantly upregulated under red light treatment. Therefore, fucoxanthin biosynthesis in phaeodactylum tricornutum under treatments with light quality of different colors is indicated by the expression levels of genes related to fucoxanthin biosynthesis, and it is possibly related to photosynthesis.