In this study, the authors propose the application of the Phase Locked Loop (PLL) technique to the control of light system. This one is destined to test the light effect on the evolution of the plants and the production improvement inside an agricultural greenhouse. In light control using PLL technique, the loop operates around a central frequency of which it departs very little, what permits to study it by the approach of continuous linear systems. The author has developed the modeling of the PLL light control and presented a comparative analysis of the phase control system results gotten by the experience, the theory and the simulation. The Performances of achieved system have been also tested experimentally.