Abstract: This article primarily summarizes the research achievements of metamorphism of lignite in recent years,and analyzes factors which impact the process that lignite changed into bituminous coal,especially the conditions.Temperature,pressure and time as external physical and chemical conditions play a key role in the metamorphic process of lignite coal,of which temperature is the most important condition.Radioactivity and contemporaneous structure are also having an impact on the metamorphic process under specified circumstances.Besides,the differences of coal forming plants,as the internal factors,should not be ignored.