Shouqu wetland of Yellow River plays important roles in the ecological security of the lower reaches of Yellow River. By the method of replacing time series with spatial sequence, an investigation was made on the changes of plant species diversity in the process of the natural degradation of the wetland. A comparison was also made to study the effects of artificial drainage on the plant species diversity. The results indicated that in the degradation process of Shouqu wetland, i.e., from swamp to swamp meadow, to alpine meadow, and to steppe meadow, the dominant plants followed the pattern of hygrophytes being gradually replaced by mesophytes and xerophytes, richness index and diversity index were increasing while dominance index was decreasing, and evenness index decreased first and increased then. The species diversity had an overall increasing trend. After artificial drainage, the proportion of poisonous weeds in the plant community increased, resulting in the increase of richness index and diversity index, slight decrease of evenness index and dominance index, and gradual decrease of Sorensen index. Artificial drainage made the habitat drying, which provided a chance for some mesophytes to invade, resulting in the increase of diversity index and richness index and the decrease of evenness index. On the whole, artificial drainage increased the plant diversity in the community, but the increase accompanied with increasing poisonous weeds, and thus, led the Shouqu wetland degraded into weed type wetland.