The monthly changes of species diversity and interspecific associations of herbaceous layer were studied in a Pinus tabulaeformis forest located on sandy site in Chifeng.The objective of the study was to obtain a better understanding of the structure and the successional dynamics of herbaceous species in that forest.The results of the study should provide a reference for scientific management,biological protection and sustainable utilization, especially a theory basis relating to maintain the natural structure and function and accelerated restoration of degenerated ecological system of Pinus tabulaeformis forests on sandy sites. Altogether 100 sample plots of 1 m×1 m were systematically distributed in a 4 hm2 experimental area. In each of these plots, the abundance, coverage and individual height of each herbaceous plant were assessed at regular monthly intervals from June to September in 2009.Up to 32 herbaceous species, which belong to 16 families and 24 genera, were investigated in this research.A range of different indices were used to define the biodiversity of the community.The number of species and the Gleason index were the greatest in August.The Simpson index, Shannon-Wiener index and Evenness index were greatest in June and July, while the lowest values were found in September. In order to analyze the relationships among the species, including the overall association and the interspecific association in the herb community, a series of techniques, including analysis of variance and the χ2 test, were used.The overall association among herbaceous plants was positive in August, but negative in June, July and September. The overall association strength among herbaceous species gradually decreased from June to September, and changed from negative to non-significant associations.The non-significant overall associations of the herbaceous species indicated that there was a certain link among the herbaceous species,but the independent distribution patterns still existed.The overall association of a certain herbaceous community was constantly changing with time,there was no single change from negative association to positive association,or from correlation to noncorrelation. The overall association of a certain herbaceous community was constantly changing with time. there was no single change from negative to positive association, or from significant to non-significant correlation. The overall association among the community varied with changes in habitat, involving changes in moisture or radiation. An interspecific association analysis of the herb species presented for every month, revealed that the ratios of positive to negative associations ranged from 0.379 to 0.558 in our investigation period (from June to September). The number of species pairs with a negative association was larger than that with positive associations in each month of the investigation period. In our investigation period, the interspecific associations between species pairs were also changing with time. This is an indication that with the dynamic development of the different species during the growing season, the community composition has changed dynamically. The decline of one species leading to the improved growth of another. The community functioning and the relationship among the different species also changed during the observation period. These changes had an influence on the interspecific associations. A positive association in a particular species pair indicates similar resource usage and niche overlapping. A negative association in a species pair indicates repellency between the two species, which may be the result of long-term adaption to different environments, different resource usage, and which may explain the niche separation.