In this paper, we study the blow-up analysis of an affine Toda system corresponding  to minimal surfaces into S^4. This system is an integrable system which is a natural   generalization of Liouville equation, sinh-Gordon equation and Tzitz\'{e}ica equation. By exploring a refined blow-up analysis in the bubble domain, we prove that the blow-up values are multiple of $8\pi$. This is a joint work with Prof. Guofang Wang.