Four high entropy alloys of FeCrMn_1.3NiAl_x (x= 0, 0.25, 0.5, 0.75) with different Al content were smelted by vacuum magnetic suspension method. The effect of Al content on the microstructure and passivation behavior of the as-cast alloys FeCrMn_1.3NiAl_x in 0.5 mol/L H₂SO₄ solution was investigated by means of XRD, FE-SEM, SKPFM, EPMA and potentiodynamic polarization technique. The results show that the FeCrMn_1.3NiAl_x (x=0, 0.25) alloy presents a two-phase microstructure of fcc (Fe-Mn-Ni)+bcc (Fe-Cr-Mn). The Fe-Cr-Mn phase of bcc structure has higher electrochemical activity, and therewith is preferred to dissolve, which results in two obvious passivation-potential and -current peaks. As the Al addition reaches x=0.5, the fcc phase almost entirely disappeared in the alloy, therefore the alloy composed of the bcc Fe-Cr-Mn phase with uniformly distributed particles and strips of b2 phase (Al-Ni-Mn). Due to the difference in electrochemical activity of the above two phases is not much, so that, the alloy present only one passivation potential and passivation current peak. Meanwhile the new hard ρ-phase of higher electrochemical activity emerges in the alloy, which resulted in lower passivation potential and smaller passivation current density. The ρ-phase disappears for the alloy with x=0.75 Al, correspondingly, the low passivation potential peak also disappears. The addition of Al can affect the passivation behavior of FeCrMn_1.3NiAl_x high entropy alloys in 0.5 mol/L H₂SO₄ solution by changing the microstructure, composition and distribution of phases in local-areas, so as the electrochemical activity of FeCrMn_1.3NiAl_x high entropy alloys. With the increase of Al, the passivation current density of the alloy increases due to the poor passivation performance of b2 phase, which in turn reduces the passivation performance of the alloys.