Abstract: The service life of metalworking fluids can be shortened by the existence of microorganism, and it is necessary to explore the composition of microbial communities in metalworking fluids. Three different methods were measured to determine the best one to separate microorganisms from metal working fluids. The concentration of microorganisms can be increased by the mikrocount combi method which has the optimal separation result. Under the help of Illumina MiSeq high-throughput sequencing, the composition of the microbial diversity of metalworking fluid samples at the 6 levels of phylum, class, order, family, genus, and species were completed, respectively. Moreover, bacteria were detected in six groups of samples, while fungus were discovered in only two groups. Meanwhile, it was easier for bacteria to thrive in metalworking fluids than fungi, and fungi only existed in samples with high bacterial contamination. A total of 2 phyla, 2 classes, 5 orders, 6 families, 10 genera and 14 species of bacteria were detected in all samples, while 4 phyla, 8 classes, 10 orders, 14 families, 15 genera and 17 species of fungi were also detected, which means that the fungal diversity is more abundant. Citrobacter_freundii_g_Citrobacter, unclassified_g_Citrobacter, unclassified_f_Enterobacteriaceae were identified as the dominant bacteria, and most of the bacteria detected were Gram-negative. The composition of the metalworking fluid will affect the type of bacteria. All detected bacteria can destroy the stability of the metal working fluid by different ways, which shortens its service life. The dominant fungi were unclassified_k_Fungi and Fusarium_petroliphilum. The health of operators will be harmed by metalworking fluids with microbial contamination.