The Intergovernmental Panel on Climate Change (IPCC) claims that air temperature will increase by 2.0-4.5 ℃ by 2100. Soil microbial communities are very sensitive to temperature and likely exert a dominant influence on the net C balance of terrestrial ecosystems by controlling organic matter decomposition and plant nutrient availability. Therefore, studying the responses of soil microbial community composition to warming is very important to predict the changes in soil microorganism and soil nutrition cycling under climate changes. Tundra is observed to warm more rapidly. Open-top chambers (OTCs) were established to simulate warming on Tundra ecosystem of Changbai Mountain. According to HOBO Data Loggers, soil temperature and soil water content were increased by 1.6 ℃ and 0.03 m³/m³, respectively. After two growing seasons (from June to September) of temperature increase experiment by OTCs, we collected soil samples in July, August and September of 2011 and measured soil microbial community structure. Phospholipid fatty acid (PLFA) analysis was used to examine the structure of soil microbial community. The results showed that warming did not change soil basic properties. The PLFA fingerprints showed that the relative abundance of PLFA markers of bacteria and fungal were higher, 52.2%-57.3% and 38.7%-45.4% of total PLFAs throughout the growing season, respectively. The relative abundance of PLFA markers of Gram-negative bacteria was lower, 7.5%-11.9% of total PLFAs. However, warming resulted in 16.1% decrease in the relative abundance of total PLFAs, and 21.2% increase in the ratios of Gram-positive to Gram-negative bacteria. There was no significant difference in the relative abundance of PLFA markers of bacteria, fungal, Gram-positive bacteria, Gram-negative bacteria, and the ratios of fungal to bacteria between warming OTCs and control plots. In addition, the seasonal dynamic changes of bacteria, Gram-positive bacteria, Gram-positive to Gram-negative bacteria and fungal to bacteria were observed except for fungal and Gram-negative bacteria. In the OTCs and control plots, the relative abundance of total PLFAs, bacteria, Gram-positive bacteria and Gram-negative bacteria were higher in July and August than that in September. The ratio of fungal to bacteria was the highest in September. Analysis of the PLFA data using principal component analysis (PCA) showed that the first two principal components accounted for 85.4% of the total variance. The relative changes in fungal and Gram-negative bacteria were very obvious according to PCA throughout the growing season. Redundancy analysis (RDA) was used to finger out which environmental factors changed the relative abundance of total PLFAs and the ratios of Gram-positive to Gram-negative bacteria. RDA showed that the ratios of Gram-positive to Gram-negative bacteria positively correlated to soil temperature, and total PLFAs negatively correlated to water content, which indicate that the changes in total PLFAs and the ratio of Gram-positive to Gram-negative bacteria directly or indirectly caused by warming change. In summary, warming significantly changed the relative abundance of total PLFAs and the ratios of Gram-positive to Gram-negative bacteria, indicated that warming caused significant dissimilarities in soil microbial community structure in warming plots after two growing seasons of experimental increase in temperature by OTCs.