I found in some article (analytic methods in algebraic geometry by Demailly) that the following formula holds
$\int_{B(0,r_0)\subset\mathbb{C}^n}\frac{|\sum a_\alpha z^\alpha|^2}{|z|^{2\gamma}}dV(z)=\mathrm{Const.}\int^{r_0}_0(\sum|a_\alpha|^2r^{2|\alpha|})r^{2n-1-2\gamma}dr$,
where $\gamma$ can be any positive real number and the series $\sum a_\alpha z^\alpha$ is just an expansion of a holomorphic map on $\mathbb{C}^n$. It is said that it holds by Parseval formula but I cannot find an appropriate version of Parseval formula. If I can know what kind of formula I should use, it will be appreciative.
If $S_{2n-1}\subset C^n$ is the unit sphere, if $d\theta$ denotes the uniform measure on $S_{2n-1}$ and $\theta^{\alpha}=\theta_1^{\alpha_1}\ldots\theta_n^{\alpha_n}$ then $\int_{S_{2n-1}}\theta^{\alpha}d\theta=0\ \ (*)$ if $\alpha\neq 0.$ Since $dV(z)=r^{2n-1}dr\, d\theta$ you just write $$|\sum_{\alpha}a_{\alpha}z^{\alpha}|^2=(\sum_{\alpha}a_{\alpha}r^{|\alpha|}\theta^{\alpha})\times \sum_{\alpha}\overline{a_{\alpha}}r^{|\alpha|}\theta^{-\alpha})$$ Now, expand, integrate with respect to $\theta$ and use (*) to get the desired result.