Three types of asphalt were chosen to carry out foaming experiments. Problem of evaluation of asphalt foamability with expansion ratio and halflife was analysed. The calculating method of foamed index and its applicability as evaluating index for asphalt foaming characteristics were also investigated. The results indicate that better foamability was not necessarily the direct result of higher asphalt temperature. It lacked theoretical basis to use expansion ratio and halflife to determine the optimal water content. Twophase fitting with different curves to the decay process of foamed asphalt could achieve more accurate foamed index. However, higher foamed index did not necessarily mean better foamability. It is suggested that foamed index be combined with expansion ratio and halflife to evaluate and design foamed asphalt. On the basis of the study in this paper, an optimal design for asphalt foamability was put forward.