Over the last 200 years, the mass burning of fossil fuels has caused excessive carbon dioxide releasing into the atmosphere, resulting in the global warming. This environmental problem has aroused people's concern, and it is urgent to exploit a new kind of sustainable energy. Comparing with other green energy, researchers believe that microalgae can not only convert CO_2 into organics through photosynthesis, and it doesn't occupy agricultural lands, which has a good development prospect. This review introduced the domestic and foreign research of microalgae starch, starch biosynthesis metabolism, the methods to improve starch accumulating, as well as at the molecular level knowing the metabolism pathway of starch synthesis and regulatory genes, in order to gain lots of starch to produce bioethanol, providing new energy to human.