Abstract: Up to now the energies selection principle for multi-energies gamma-ray transmission technique was ambiguous, and the research object of this paper was to extract a energies selection principle for it. The relationship between measurement precision and ill-equations has been demonstrated in this paper, and a energy selection principle has been got based on these analysis, which is to chose the energy parameters by determining the condition number of the attenuation coefficient matrix, and the condition number for engineering application proposal is less than 100. Meanwhile, a series experiments based on U and Zr mixture have been used to verify the principle. The results show that smaller condition number of the attenuation coefficient matrix lead to high measurement accuracy, and the relative measurement accuracy can be controlled within 5% based on the precondition of condition number smaller than 100.