[Objective] The effects of the enclosing on vegetation composition and community complexity were examined in grassland of Yanchi County, Ningxia Hui Autonomous Region, in order to provide the theoretical basis for vegetation restoration and grassland management in semi-arid area. [Methods] By quadrat method, the vegetation was investigated from 2003 to 2013 with respect to the impacts of different fencing time on vegetation composition and three kinds of complexity index(total complexity, disorder-based complexity, and structural complexity). Three processing methods of study region were adopted: old enclosure area, new enclosure area and control area. [Results] Artemisia ordosica and Salsola ruthenica communities gradually converted into Heteropappus altaicus community in the old enclosure area, and Artemisia ordosica was dominant species in new enclosure and control area. The biomass and coverage in the old enclosure area were less than those in the new enclosure area and control area; enclosure increased disorder-based complexity and structural complexity, thus which made the total community complexity increase. [Conclusion] The long-term enclosure is not conducive to vegetation recovery. The high peak and low peak values of each complexity index appeared regularly with the cycle of five-year. So, 5-year is appropriate enclosure period for grassland in the semi-arid area.