Abstract: Objective：To investigate the reasons of unacceptable results and corrective measures adopted in external quality assessment (EQA)for blood gas and acid-base analysis. Methods：The reasons of unacceptable results and corrective measures for three EQA testing events of blood gas and acidbase analysis in 2016 were reported through EQA system based on web which was developed by National Central for Clinical Laboratories. The responses were divided into seven major groups, including EQA samples, errors in reporting results, methodology, equipments, techniques, EQA evaluations and unexplainable results after survey. Results：The disqualified rates of EQA survey on blood gas and acidbase analysis were ranged from 0.5% to 13.1% and reporting rates of disqualification causes were ranged from 45.8% to 69.0% (except for the groups less than 20 laboratories). In the reasons for unacceptable results technological defects (35.9% to 37.0%)were mainly associated with inappropriate specimen handling and/or storing, reagents and calibration problems. The defects of equipments (24.4% to 27.9%)included mainly the malfunction and failure to adhere to scheduled instrument maintenance procedures. The errors in reporting results (12.2% to 19.7%)were mostly transcription errors and reporting wrong codes. The unexplainable results after survey account for 8.7% to 9.6%. The methodological defects (8.1% to 11.8%)were largely attributed to inadequate training and quality control method. The defects of EQA evaluations (0.8% to 3.3%)were all due to inappropriate grouping. The categorizations of the problems in the three EQA testing events were similar. The most corrective measures were appropriate, in which re-education and training for staff and improvement in instruments, reagents, internal quality control, calibration and process of reporting results were included. Conclusion：The analysis and classification for reasons of unacceptable EQA results should be helpful for laboratories in identifying opportunities for improvement and adopting corrective measures in time.