We propose a novel pulse amplification model to obtain the effective thermal equilibrium time (${\tau }_{\mathrm{ETE}}$) for Yb:YAG crystal, where ${\tau }_{\mathrm{ETE}}$ has impacts on the gain recovery and energy extraction. A test amplifier is set up to measure the input fluence, output fluence, and pulse duration. By fitting the numerical values to the experimental data, the effective thermal equilibrium of Yb:YAG crystal at room temperature is found to be between 60 and 120 ps. To our best knowledge, this is the first time that an exact range of effective thermal equilibrium time for Yb:YAG is reported.