Abstract Purpose The purpose of this study is to determine the uterine cavity microbial composition in unexplained recurrent spontaneous abortion (URSA) patients and to understand the correlation between URSA and uterine cavity microbiota. Methods Using the double-lumen embryo transfer tubes to collect uterine cavity fluid samples from pregnant women in their first trimester. 16S rRNA sequencing was conducted to analysis the composition and abundance of the microbiota in samples. Results We enrolled 10 URSA cases and 28 induced miscarriage cases in their early pregnancy. Microbial communities were detected in all samples of URSA group (100%, n = 10) vs. none of control group (0%, n=28). Two most dominant microbes are Lactobacillus and Curvibacter. Conclusion This study showed Lactobacillus and Curvibacter dominated colonizing in uterine cavity of URSA patients during early pregnancy and associated with URSA. Changes of dominant microbiota will lead to bad pregnancy outcome.