In order to evaluate the influence of dark enclosed environment of ship ballast tanks on the growth of green algae, the study was carried out at five different initial densities (5 cells/mL, 50 cells/mL, 5×10² cells/mL, 5×10³ cells/mL, 5×10⁴ cells/mL) to investigate the relative growth inhibition rate of Platymonas helgolandica under dark conditions and the population response at different growth periods and initial density treatments. The light recovery study of dark-treated Platymonas helgolandica during the dark period was conducted to explore the differences in the population density changes of Platymonas helgolandica without darkness and after darkness at different initial densities. The results showed that after the dark-treatment,low-density (3 cells/mL) Platymonas helgolandica can still reach a higher population density (1×10³ cells/mL) after two weeks of light recovery. For densities of 3 and 3×10⁴ cells/mL,the population resilience of the dark-treated Platymonas helgolandica was weaker than that of the untreated group and there was a significant difference (P<0.05).