Series of Sr2Fe(1+x)MoO6 (x = 0, 0.03, 0.05, 0.08) ceramics have been synthesized by introducing metal Fe at grain boundaries. The increasing of Fe segregation changes the Fe/Mo anti-site defect (ASD) and decreases grain boundary (GB) strength significantly. Fe/Mo ASD contributes to low-field magnetoresistance (LFMR) negatively, but GB positively. When x <= 0.03, the ASD content of the maternal Sr2FeMoO6 (10%, x = 0) decreases to 8% (x = 0.03), which compensates the lowering of GB strength partially, and hence a slightly improved LFMR behavior is observed. However, when x <= 0.03 (x = 0.05, 0.08), Fe impurity bring about heavy ASD content and decreasing GB strength, so the LFMR property is suppressed greatly. The magnetoresistance behavior has been explored by a fitting model further. We believe that, our work may provide a facile and effective strategy for adjusting magnetoresistance response by tuning B-site ordering and GB strength of the double perovskite.