The X-ray light curve of Sw~J1644+57 indicates this event would be due to a tidal disruption. The lightcurve shows large amplitude fluctuation. As proposed by Lyubarskii (1997), the aperiodic variability observed in the Galactic X-ray binaries and active galactic nuclei is likely from the fluctuation of the viscous parameter in their disks. We explain the significant fluctuation of the late X-ray lightcurve (t>106 seconds) of Sw J1644+57 with this model. We assume the stochastic variations in the viscous parameter featuring as α(R,t) = α0 [1+β(R,t)], where the time-scale for varying β(R,t) is set as ten times of the dynamic time-scale for disk at the radius R (Janiuk & Misra 2012). Based on the simulation results of Lodato et al. (2009), we describe the fallback behavior of the tidal disruption as Ṁfb ∝ {[(t - tb)/tfb]κ n + [(t - tb)/tfb]5n/3}−1/n for t > tb and Ṁfb=0 for other situations, where κ=10.0, n=0.5, tfb=103τ, and tb=102τ in which τ=2π(Rf3/GMBH)1/2 and Rf=5rg is the pericentre distance. Figure 1 compare the power-density spectra (PDS) derived from the observed and our simulated lightcurves. It is found the our simulations are well consistent with the observations.