In this paper, the effects of Cr on segregation of S or Cl atoms at the a-Fe S5(210) grain boundary (GB) are studied using a first principles method, CASTEP. In the absence of Cr, S can be trapped at the interstitial site of the Fe GB. Cl is not apt to be segregated at the GB. The presence of Cr acts as an enhancer, and can alleviate the detrimental effect of Cl and S. Furthermore, increasing the Cr content has a more effective impact on Cl. Based on the segregation energy analysis, it is also found that Cr can effectively prohibit S from segregating to the GB and therefore eliminate the embrittlement of the GB induced by S; however, it has little effect on Cl. Cr prevents the expansion of interfacial bonds Fe-Fe or Fe-S/Cl for S, and Cl segregating in Fe GB even exhibited shorter bond lengths with increasing Cr content. Therefore, Cr alleviates embrittlement or corrosion effect of S and Cl in Fe GB. Analysis of densities of states further indicates that Cr should alleviate the detrimental effect of S and Cl, enhancing the cohesion of a-Fe GB.