Abstract: With the extension of the life of spacecraft in orbit, the molecular contamination has more and more prominent effect on the safety and reliability of space optical systems. In this paper, the origin of space molecular contamination and its effect on space optical components were given firstly. And then the effects of natural and artificial environments such as temperature, ultraviolet, and laser irradiation on molecular contamination of optical components were analyzed. The evolution and morphology of molecular contamination on optical components affected by space environment were revealed. Further, the techniques to mitigate or to remove the molecular contamination from space optical components, such as ground pretreatment, surface modification, optimized system design and in-orbit active removal, were presented. The research directions, such as highly sensitive monitoring of molecular contamination, application of thermal control and molecular contamination adsorption composite coatings, efficient in-orbit removal of molecular contamination, advanced synergistic simulation equipment as well as computational simulation technology, were proposed lastly.