Abstract: The 15th issue of IYPT in 2017 is "Boiled Eggs",which requires a method to measure the doneness degree of boiled egg without destroying the eggs. In this paper,a controllable egg was cooked in a water bath. Using the difference in light transmission between raw and cooked eggs,a simple visible light transmission method was selected. Based on the SLR mechanism,the test device was designed and analysis showed a good correlation between the relative light transmittance and the maturity level of the boiled egg. By taking a light-transparent photograph of the egg and calculating the relative transmittance,the maturity of the egg can be obtained by looking up the table. An experimental device for non-destructive testing of egg maturity was built,which can achieve eight-level maturity classification of eggs with high precision and good repeatability.