In order to study the effects of different enclosure and different years on the plant characteristic values and species diversity, quadrate method was used to investigate plant growth during the growing season from 2004 to 2011 in three sample plots in the fencing area of Yanchi County, Ningxia Hui Autonomous Region.The results showed that with the extension of the enclosure years, Artemisia ordosica Krasch and Salsola ruthenica Iljin become dominated species of vegetation.Through the analysis of vegetation diversity index in recent years, peak value and low peak of each index showed a regular cycle of 5 years.This showed that the period of artificial enclosure was 5 years in arid and semi-arid region.