  Fast Blue Optical Transients (FBOTs) are luminous transients with fast
evolving (typically $t_{\rm rise}<12\ \rm days$) light curve and blue color
(usually $\rm {-0.2\ >\ g-r\ >\ -0.3}$) that cannot be explained by a
supernova-like explosion. We propose a radiative diffusion in a time-dependent
outflow model to interpret such special transients. In this model, we assume a
central engine ejects continuous outflow during a few days. We consider the
ejection of the outflow to be time-dependent. The outflow is optically thick
initially and photons are frozen in it. As the outflow expands over time,
photons gradually escape, and our work is to model such an evolution. Numerical
and analytical calculations are considered separately, and the results are
consistent. We apply the model to three typical FBOTs: PS1-10bjp, ZTF18abukavn,
and ATLAS19dqr. The modeling finds the total mass of the outflow ($\sim 1-5
{M_{\odot}}$), and the total time of the ejection ($\sim$ a few days) for them,
leading us to speculate that they may be the result of the collapse of massive
stars.
