We theoretically investigate the high-order harmonic generation (HHG) power spectra from a one-dimensional argon model atom irradiated by an infrared two-color laser beam, which are composed of 9.7 fs/800 nm laser pulses and 9.7 fs/1600 nm laser pulses. It is found that, when the relative phase equals π, the harmonic cutoff is dramatically widened from 58 eV to 316 eV compared with the case with the single color laser beam. It is also demonstrated that an ultrabroad soft-X-ray harmonic supercontinuum with a bandwidth of 93 eV can be obtained by adopting the soft-core potential and the Pschl-Tell potential calculations. In terms of the time-frequency analysis and the semiclassical three-step model, we also demonstrate that the ultrabroad harmonic supercontinuum is mainly generated from the short path. By superposing a series of properly selected harmonics, an isolated attosecond pulse with duration of 46 as is obtained. Compared with other schemes of generating single as pulses by quantum path control in two-color laser pulse, the10 fs/800 nm pulse used in our scheme enables easier manipulations in practical experimental implementations.