To investigate changes of ultrafast dynamics during irreversible phase change in phase change materials Ge2Sb2Te5, we performed high-repetition-rate singleshot pump-probe spectroscopy using a combination of time-encoding and timestretching methods. By measuring the pump-probe traces while ramping the pump intensity, we observed a clear change in the ultrafast pump-probe dynamics after the phase change. Correlation between the ultrafast dynamics in the femtosecond timescale and the amount of phase change observed in millisecond timescale indicates that accumulation of the excited states in the sample plays an important role in the acceleration of the phase change. The result clearly demonstrates the usefulness of our method, which could be applied to the investigation of multi-timescale dynamics in various irreversible phenomena. Improved signal-to-noise ratio and the variable time-window of the single-shot pumpprobe measurements were also demonstrated using a grating pair and a chirped fiber Bragg grating.