Platymonas subcordiformis, a marine green alga, was demonstrated to photo-biologically produce hydrogen when regulated by a kind of proton uncoupler CCCP (Carbonyl Cyanide m-Chlorophenylhydrazone). In this paper, hydrogen production experiments by P. subcordiformis were carried out in a torus photobioreactor equipped with a mass spectrometer and other necessary sensors so that instantaneous gas components could be measured and other successive physiological states could be well recorded. Hydrogen production performances at different cell densities were compared. We demonstrated that with six folds biomass increasing, hydrogen yield increased almost thirty times. Along with the increase of total hydrogen production yield, rate and duration of mass hydrogen production yield were also much higher. This meant that each alga cell was more efficient in hydrogen production experiments at high cell density. The energy utilization efficiency of hydrogen production from P. subcordiformis was further calculated, average energetic yield of light conversion to hydrogen energy was about 0.3%. Here, characteristics of hydrogen metabolism were also studied. By comparing hydrogen production kinetics, photosystem II activity and change trends of endogenous substrates, we gained the knowledge that photosystem II activity and substrates consumption played an important role in P. subcordiformis hydrogen production at different phases.