  We develop a theoretical approach to study the scaling of anomalous Hall
effect (AHE) in the insulating regime, which is observed to be
$\sigma_{xy}^{AH}\propto\sigma_{xx}^{1.40\sim1.75}$ in experiments over a large
range of materials. This scaling is qualitatively different from the ones
observed in metals. Basing our theory on the phonon-assisted hopping mechanism
and percolation theory, we derive a general formula for the anomalous Hall
conductivity, and show that it scales with the longitudinal conductivity as
$\sigma_{xy}^{AH}\sim\sigma_{xx}^{\gamma}$ with $\gamma$ predicted to be
$1.38\leq\gamma\leq1.76$, quantitatively in agreement with the experimental
observations. Our result provides a clearer understanding of the AHE in the
insulating regime and completes the scaling phase diagram of the AHE.
