  Attosecond x-ray pulse with known spectro-temporal information is an
essential tool for the investigation of ultrafast electron dynamics in quantum
systems. Ultrafast free-electron lasers (FELs) have the unique advantage on
unprecedented high-intensity at x-ray wavelengths. However, no suitable method
has been established so far for the spectro-temporal characterization of these
ultrashort x-ray pulses. In this letter, a simple method has been proposed
based on self-referenced spectral interferometry for reconstructing ultrashort
FEL pulses both in temporal and spectral domains. We have demonstrated that the
proposed method is reliable to reconstruct the temporal profile and phase of
attosecond x-ray FEL pulses with an error of only a few-percent-level.
Moreover, the first proof-of-principle experiment has been performed to achieve
single-shot spectro-temporal characterization of ultrashort pulses from a
high-gain FEL. The precision of the proposed method will be enhanced with the
decrease of the pulse duration, paving a new way for complete attosecond pulse
characterization at x-ray FELs.
