Shrub encroachment is the increase in density, cover and biomass of indigenous woody or shrubby plants in arid and semiarid grasslands, which become a great ecological problem in arid and semiarid region. It is one of the most important parts of research in the global change of terrestrial ecosystem. Shrub encroachment was resulted from the interactions of many factors including over grazing, fire, climate change, elevated atmospheric CO_2, changes about biotic and abiotic environment factors. Shrub encroachment could occurred a suddenly vegetation shift from a stable stage with grasses dominant to a stable stage with shrub dominant, with a great change in the composition and structure of plant community. The whole landscape fragmentation and heterogeneity increased in the processes of shrub encroachment. The light, heat, water and nutrients in ecosystem were redistributed with the establishment and development of shrub seedlings. Shrub patches gradually formed "fertile islands" with water and nutrients enrichment. The current studies about shrub encroachment were mainly concentrated on fixed-point research and factor controlled experiments with distinct locality and restrictions. We should pay more attention on the relationship of climate change and shrub encroachment, feedback of grassland ecosystem for shrub encroachment and research about strategies and countermeasures relative to shrub encroachment, model simulation might be one of important ways to study the occurrence and development of shrub encroachment even to predict the succession of ecosystem.