An attosecond x-ray pulse (AXP) with relativistic intensity is a coveted light source that has great appeal in wakefield acceleration, quantum electrodynamics in strong field physics, and nonlinear attosecond science. Here, a quantum electrodynamical (QED)-dominated coherent synchrotron emission regime is proposed for the generation of a relativistically intense AXP by using an ultraintense laser pulse with intensity of the order of interacting with a double-foil target, which is accompanied by the production of high-energy gamma () photons and an electron–positron () pair. The positron () bunch generated by the QED process provides the crucial Coulomb force for the formation and acceleration of relativistic electron sheet that is responsible for the generation of relativistically intense isolated AXP. The spectrum of the AXP exhibits a distinct plateau profile, which allows for the generation of ultra-intense attosecond pulses, of which the intensity can reach about . By applying spectral filtering, an isolated AXP with a duration of can be obtained in the transmission direction, which can reach the x-ray range with an intensity of . This will offer opportunities to investigate relativistically nonlinear attosecond science.