MDS codes have the highest possible error-detecting and error-correcting capability among codes of given length and size. Let p be any prime, and s, m be positive integers. Here, we consider all constacyclic codes of length p s over the ring R = F p m + uF p m (u 2 = 0). The units of the ring R are of the form α + uβ and γ , where α, β, γ ∈ F * p m , which provides p m (p m − 1) constacyclic codes. We acquire that the (α + uβ)-constacyclic codes of p s length over R are the ideals (α 0 x − 1) j , 0 ≤ j ≤ 2 p s , of the finite chain ring R[x]/ x p s − (α + uβ) and the γ -constacyclic codes of p s length over R are the ideals of the ring R[x]/ x p s − γ which is a local ring with the maximal ideal u, x − γ 0 , but it is not a chain ring. In this paper, we obtain all MDS symbol-pair constacyclic codes of length p s over R. We deduce that the MDS symbol-pair constacyclic codes are the trivial ideal 1 and the Type 3 ideal of γ -constacyclic codes for some particular values of p and s. We also present several parameters including the exact symbol-pair distances of MDS constacyclic symbol-pair codes for different values of p and s.INDEX TERMS Repeated-root codes, constacyclic codes, MDS codes, symbol-pair distance, finite chain ring.