An ultraintense, isolated circularly polarized (CP) attosecond x-ray pulse is often required for many application of pump-probe techniques. A quantum electrodynamics (QED) effect dominated coherent synchrotron emission (CSE) regime is proposed for the generation of an ultraintense isolated, nearly CP attosecond x-ray pulse. Due to the QED effects, the density of the target plasma increases due to Breit-Wheeler pair generation and the radiation reaction effects of gamma photons through the nonlinear Compton effect. As a result, the transparent target becomes an opaque target. Accordingly, the relativistic electron sheet (RES) can be accelerated toward a reflected direction only once. After that, the RES was pushed tempestuously forward continuously under the Lorentz force of the driving laser pulse. Therefore, an isolated, nearly CP attosecond x-ray pulse can be obtained in the reflected direction without filtering. The intensity of the nearly CP attosecond x-ray pulse can reach $2.48\times {10}^{24}\mathrm{W}/{\mathrm{cm}}^2$ with ellipticity of $0.83$, and the corresponding power can reach up to $\sim 100\mathrm{PW}$, which opens the door for nonlinear attosecond studies.

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• Laser-plasma interactions
• Radiation & particle generation in plasmas
• Ultrashort pulses