In the paper, we obtain the optimal integrability for positive solutions of the following integral system involving Wolff potentials:        u(x) = W β,γ (v q)(x), x ∈ R n , v(x) = W β,γ (u p)(x), x ∈ R n , where p, q > 0, β > 0, γ > 1 and 0 < βγ < n. Ma, Chen and Li [Advances in Mathematics, 226(2011), 2676-2699] developed the regularity lifting method and obtained the optimal integrability for p > 1, q > 1. Here, based on some new observations, we overcome the difficulty there, and derive the optimal integrability for the case of p > 0, q > 0 and pq > 1. This integrability plays a key role in estimating the asymptotic behavior of positive solutions