An attosecond light source provides an advanced tool for investigating electron motion using time-resolved-spectroscopy techniques. Isolated attosecond pulses

especially

will significantly advance the study of electron dynamics. However

achieving high-intensity isolated attosecond pulses is still challenging at the present stage. In this paper

we propose a novel scheme for generating high-intensity

isolated attosecond soft x-ray free-electron lasers (FELs) using a mid-infrared (MIR) subcycle modulation laser from gas-filled hollow capillary fibers. The multi-cycle MIR pulses are first compressed to subcycles using a krypton-filled hollow capillary fiber with a decreasing pressure gradient due to the soliton self-compression effect. By utilizing such subcycle MIR laser pulses to modulate an electron beam

we can obtain a quasi-isolated current peak

which can then produce an isolated FEL pulse with a high signal-to-noise ratio

naturally synchronizing with the subcycle MIR laser pulse. Numerical simulations have been carried out

including subcycle pulse generation

electron beam modulation

and FEL radiation processes. The simulation results indicate that an isolated attosecond pulse with a wavelength of 1 nm

a peak power of ~28 GW

a pulse duration of ~580 as

and a signal-to-noise ratio of ~96.2% can be generated by our proposed method. The numerical results demonstrated here pave a new way for generating a high-intensity isolated attosecond soft x-ray pulse

which may have many applications in nonlinear spectroscopy and atomic-site electronic processes.