Weak absorption is the main condition of conventional phase retrieval of in-line X-ray phase-contrast CT. The absorption of most samples, however, cannot be neglected in practice, it is needed to find new algorithms of attaining quantitative information. The phase retrieval algorithm of phase-attenuation duality (PAD) is used to realize the quantitative imaging of different density parts in a mixed contrast sample, and the digital simulation and experimental investigation are carried out in order to prove its feasibility in this paper. The simulated results show that the reconstructive errors of three materials are less than 1%, and the values decrease with the increase of refractive index. The experiment was carried out in the X-ray imaging and biological application beam-line of Shanghai Synchrotron Facility, and the results show that the PAD method can attain the sample quantitative information and the reconstructive accuracy is relatively low compared with that of simulation. Through the analysis of experimental data, ring artifact is mostly the primary cause. One single projection data set is used only in simulation and experiment. So the PAD approach can be applied to the quantitative information research of mixed contrast objects. In addition, because of the relatively low radiation dose, it should be suited to investigate the quantitative phase-contrast CT of biomedical sample with soft tissue and bone together.