Abstract: It is difficult to run casing in and the cementing quality is poor due to small annular gap in drilling or workover of deep and ultradeep wells. Flush joint casing can solve above problems. It connects with threads on both ends of pipe body, which allows a smaller casing O. D. and increases annular gap to facility casing running in and improve cementing quality. This paper presents flush joint casing classification, structural feature, technical difficulty in design, performance and evaluation, main advantages and application.