Abstract: Monte-Carlo method was used to simulate the dual-detector density logging. Energy spectra, the relation of detector count ratio and formation density and the spine-and-rib plot were calculated and compared with experimental results, and it is proved that the simulation is valid. Based on these results, the effect of density logging instrument structure (for example, short spacing and short-spacing collimator angle) on logging was studied. The research results prove that the requirements of logging sensitivity and compensated density precision are met when short spacing is 18-20 cm and short-spacing collimator angle is 45°