In order to investigate ceftazidime/avibactam (CZA) resistance characteristics and mechanisms of bacteraemic Enterobacterales strains that had not been treated previously with CZA, 9708 strains were collected from 43 hospitals in 18 provinces across China from January 2019 to June 2020. The minimum inhibitory concentration (MIC) values of CZA in 165 (1.70%) strains were ≥8/4 mg/L. Ten (6.06%) CZA-resistant strains without metallo-β-lactamase production were obtained from the individuals without prior exposure to CZA, including six Escherichia coli isolates, three Klebsiella pneumoniae isolates and one Enterobacter cloacae isolate. Whole-genome sequencing revealed that ECB88611, ECB142593 and ECB144539 had encoded disrupted OmpF loss of function. OmpF of ECB126041 had a 2_9 MKRNILAV deletion; OmpK35 of three K. pneumoniae isolates harboured amino acid fragment deletions from positions 1 to 38; and ELB117287 had encoded disrupted OmpF. The G132D amino acid substitution of OmpC of ECB88611, ECB142593 and ECB144539, and the 134_135GD insertion of OmpK36 of three K. pneumoniae isolates were predicted to alter ceftazidime permeability. 333_334 YRIK or YRIN insertions occurred in PBP3 of six E. coli isolates. The relative expression of bla in KPB125108 was 4.527 ± 0.2166 times higher than the control strain, and the relative expression of acrF in six E. coli isolates was 2-3 times higher than the control strain. The addition of phenylalanine-arginine-β-naphthylamine at 100 mg/L decreased the MIC values of CZA against nine strains significantly. In conclusion, the antimicrobial resistance mechanisms in 10 isolates included increased expression of bla, non-functional OMPs, upregulation of efflux pump activity, and variants of PBP3. Most of these mechanisms affected the antimicrobial activity of CZA by impeding ceftazidime.