In this paper, the author establishes a prior estimate of the radially symmetric solutions, and apply a Poho{z}aev-type identity to the research on the existence of solutions for the polyharmonic equation
{(-Δ)mu=|x|αup-1, u>0, in B,
Diu|∂B=0, |i|≤m-1
where m ∈N, B is the unit ball in Rn with n≥2m+1, α>0 and p>2. Moreover, by making use of the Blow-up analysis, the asymptotic behaviour of the least energy solutions for above equation is obtained when α is fixed and p→2n/(n-2m) .