This study aimed to prepare a composite bar by thermal simulation experiment mechanism considering 316L/EH40 composite with Fe-Co-Ni interlayer as the research object. First, the barrier property of the Fe-Co-Ni interlayer was verified. Then, the effects of coating temperature, interlayer vacuum degree, and mold temperature on the coating ratio of Fe-Co-Ni interlayers, melt solidification characteristics, and shrinkage cavity were analyzed using the ProCAST simulation software. The results showed that an appropriate increase in the coating temperature could effectively improve the coating ratio and reduce the shrinkage cavity. The interlaminar vacuum degree increased the interlaminar coating ratio and reduced the shrinkage cavity; however, the shrinkage cavity rate increased when the interlaminar vacuum degree was too high. Increasing the mold temperature promoted the coating ratio and reduced the shrinkage cavity. The area where the interlayer could be completely coated was significantly reduced with the decrease in the interlaminar thickness.