Abstract: The author conceives that the subjects on the .analysis of the most economical parameters of steam-gas combined cycles so far published may be divided into two different groups,both having some defects. The first group is the so called direct computation method from many variables, its chief defects are very complicated in computaion and its highly differculty of summary conclusions from special cases into more general ones.The second group includes those of theortical derivations with different idealizing assumptions, their defects are theory and actual conditions not very closely related. The present subject is an intermediate method between the above two, it introduces a x cycle Substitution, the original steam cycle in the combined cycle can be entirely separated from the gas part without any modification in computation, thus the method of analysis is greatly simplified and some more general conclusions can also thereby be drawn out.