The study of light-dependent growth of butch culture Phaeodactylum tricornutum has been carried out. Based on the developed mathematical model of the true absorption spectrum, an express method for determining the concentration of photosynthetic pigments without interfering with the growth process of the culture was proposed. In the exponential phase at an irradiance of 120 μE·m-2·s-1, the maximum specific synthesis rates of chlorophylls a and c were determined, which were 1,4 times higher than the specific growth rate of the culture and amounted to 0,3 day-1. On the eighth day of the experiment, a kink in the growth curve was observed, which was expressed as a decrease in both growth rate and chlorophyll production. At the transition to the linear growth phase, the maximum productivity of Pheodactylum was 0,15 g·l-1·day-1, and chlorophyll production was 3,44 and 2,85 mg·l-1·day-1 a and c, respectively. The dependence of the integral light absorption coefficient on chlorophyll a concentration was obtained, which is described by the Bouguer-Lambert-Bera law with a sufficient degree of accuracy; the specific light absorption coefficient was 0,10 m2·g-1 dry matter and 0,008 m2·mg-1 chlorophyll a. Comparison of the results obtained with literature data showed that at irradiances of 120 μE·m-2·s-1 and 602 μE·m-2·s-1 the specific rates of chlorophyll a synthesis are the same, and the maximum specific growth rate of Ph. tricornutum culture increases proportionally with increasing light intensity from 0,23 to 0,91 day-1. The results obtained indicate that chlorophyll a synthesis is determined not by the effective light intensity, but by the amount of reserve biomass accumulated during the previous light period.