Abstract: In order to provide a scientific basis for litchi quick and nondestructive detection and classification based on litchi internal quality, the nondestructive testing approach of litchi soluble solids content was studied. First, in the light of litchi hard and rough pericarp, results of diffuse reflectance and transmission experimental methods were compared. And then a variety of pretreatment methods were used to process diffuse transmission spectroscopy. The successive projections algorithm combined with correlation coefficient method was used to select optimal modeling wavelength. At last, partial least squares model and neural network model were built to predict the SSC of litchi. The results showed that diffuse transmission method was better than diffuse reflectance method for litchi spectrum acquisition; After variable compression, 11 preferred wavelengths were extracted, only accounts for 2.2% of the total 500 wavelengths; Based on the 11 wavelengths, the correlation coefficient of the BPNN model for predicted SSC of litchi was 0.867, and the root mean square error was 0.370%. The results indicate that the nondestructive detection for litchi soluble solids content based on diffuse transmission method is feasible.