The crystal polymer's correlation function of threephase structure with linear electron density interphase layers is calculated.The corresponding relation about invariant,interphase layers' thickness and scatter figure are found.The conclusions are compared with that G.R.Strobl got in the extension from two-phase model to three-phase model.Strobl's errors are pointed out and corrected the errors in the extension.The authors get long period,lamellar thickness,invariant and interphase layers' thickness from the correlation function figure of three-phase model,but the conclusion is that the definitions of lamellar thickness and invariant are not the same as those in the two-phase structure.