In order to study the correlation between the fucoxanthin content of Phaeodactylum tricornutum and its photosynthetic physiological indexes, different monochromatic light-emitting diodes were used as light sources to treat P. tricornutum. The fucoxanthin content, photosynthetic physiological indexes, and the expressions of photosynthesis-related genes were detected, and the correlation among the indicators was examined. The results show that purple light can significantly promote fucoxanthin and chlorophyll a production of P. tricornutum (P<0.01). Under red light, the value of Y (II), maximum relative electron transfer rate (rETRmax), and alpha readings of the algae cells were significantly higher than those of the control group (P<0.01). Under yellow light, the nonphotochemical quenching value of the algae cells is at its lowest, which was 63.45% lower than that of the control group (P<0.01). The results of the quantitative reverse transcription-PCR show that blue light is more conducive to the expression of the rbcL gene. Under red, blue, green, and purple light conditions, the expression level of fcpB is significantly increased compared to the control group (P<0.01), and the expression level is increased by 5.4 times under red light. Combined with Pearson analysis, the content of fucoxanthin was positively correlated with the rETRmax and the expression of the fcpB gene, and negatively correlated with the expression of the rbcL gene. In conclusion, the synthesis and accumulation of fucoxanthin can be promoted by improving the photosynthetic efficiency of P. tricornutum, which can be done by regulating the parameters of rETRmax and the expression of the fcpB gene at different wavelengths.