Foamed mixture lightweight soil (FMLS) is a new environmental protection geotechnical material. It has been applied to embankment filling. However, the issue of optimum casting thickness of each layer FMLS has not been solved yet until now. In this study, a self-developed instrument is used to measure the strength of the FMLS at different depths during construction. The testing principles and steps are given; and the judgment criterion of the optimum casting thickness is determined. Through an engineering case, a method is proposed for determining the optimum casting thickness for FMLS as embankment filling material. It is suggested that the optimum casting thickness should be controlled within 500 mm for test section. This procedure is beneficial to control construction quality of FMLS as embankment filling material.