Abstract: As oil and gas exploration continuously progresses, much attention is paid on the research of the optimization design and measurement methods of the next-generation controllable source X-ray density logging tools. Taking the existing ultra-slim gamma-ray density logging tool for an example, it is re-designed by replacing gamma-ray source with X-ray source. The research mainly focuses on the following points:(1) to simulate the generation mechanism of X-ray with different energies; (2) to study the optimal spacing of X-ray density logging tool when comprehensively considering the account detection efficiency, density measurement sensitivity, detection depth and other factors; (3) to carry out X-ray density measurements using multi-variable forward and inversion methods, and further compare with gamma density logging results. The results indicate that the performance of X-ray density logging tool can equal to or even exceed that of gamma-ray density logging tool by adjusting the source-detector spacings of near and far detectors to 110 mm and 290 mm, respectively. Compared with gamma-ray density logging, X-ray density logging is characterized by higher formation sensitivity, vertical resolution and formation density measurement accuracy. In addition, X-ray density logging tool can improve measurement accuracy of photoelectric absorption cross-section index by 0.15 b/e in condition of heavy mud cake. In conclusion, these findings provide certain technical support for the prospective design of X-ray density logging tool and the accuracy measurement of formation density.