The carbon cycle of grassland ecosystem and its impact factors has become a study focus in the field of ecology, geology, botany and agronomy due to drastic global change in recent years. The present paper introduces the process of carbon cycle of grassland ecosystem and discusses the circulation of carbon among atmosphere sink, vegetation sink, herbivores sink and soil sink. Furthermore,the influences of climate factors,soil factors,CO_2 enrichment and human activities on carbon cycle were also analyzed here. The main problems and prospects of the study on grassland ecosystem carbon cycle are proposed.