We have studied the ultrafast dynamics of two-photon-pumped amplified spontaneous emission (ASE) from a single crystal by the time-resolved fluorescence upconversion technique. With the increase of two-photon pump intensities, the emission decay time is dramatically shortened by 30 times (from $3\mathrm{ns}$ to ~87 ps), and the energy migration rate is acutely enhanced when ASE occurs. The stripe length is also found to play an important role in the formation of the ASE. Meanwhile, the gain coefficient is evaluated to be $15{\mathrm{cm}}^{-1}$ for $560\mathrm{nm}$ at an excitation intensity of $2.3\mathrm{mJ}/\text{pulse}/{\mathrm{cm}}^2$ by the variable stripe length technique.