During the usage of railway transport, the complex environment leads to the decrease of durability of concrete structure. The effects of slag content, water-binder ratio and stray current voltage intensity on the microstructure of slag cement based composites under the coupling effect of stray current and soft water leaching were studied. The phase composition, Ca(OH)2 content, microstructure and pore structure of slag cement based composites after 90 d calcium leaching were analyzed by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and mercury intrusion method. The results show that the larger the slag content, the less Ca(OH)2 content in slag cement based composites, the lower the porosity, and the more uniform and dense the structure. With the increase of stray current voltage intensity, Ca(OH)2 and AFt content in slag cement based composites decreases, and the phase boundaries of AFt and Ca(OH)2 become blurred. C-(A)-S-H content in the material increases slightly, and the porosity decreases slightly. With the increase of water-binder ratio, Ca(OH)2 content in the material decreases, the residual phase content decreases, the microstructure becomes loose and porous, the pore size structure coarsens as a whole, and the porosity increases.