The transmittance characteristic of three-slabs whose configuration is glass-semitransparent liquid-glass was analyzed based on ray trace method，and transmittance ratio calculation model of semi-transparent liquid was developed.The unique value range and multi-value range of double-thickness model of thermal radiation physical property measurement of semi-transparent liquid with transmission method were discussed，and the application range of the inverse model was attained.The optical constants of heptane attained in the references were selected as the true values，and the spectral transmittance ratio of the semi-transparent liquid based on the direct model simulation were regarded as the experimental values.The optical constants of heptane were achieved by the inverse models.Good agreement was obtained between the retrieved optical constants of heptane and the experimental data attained in the references.Then the influences of measurement error on the inverse results were also investigated.