This paper reports the PANI (polyaniline)-packaged three-dimensional (3D) alpha-Fe2O3 nanotubes P/N heterojunction, which was constructed successfully though a facile hydrothermal reaction without adding any surfactant followed by in-situ chemical polymerization in the liquid phase. The structure and morphology of the products were characterized by means of FESEM, XRD, TEM & HRTEM, TGA and FT-IR. Gas sensing test results found that, the PANI/alpha-Fe2O3 nanocomposite sensor exhibited obviously higher response, lower working temperature (180 degrees C), fast response and recovery speeds, good selectivity and excellent stability to acetone gas, indicating its promising application for high-performance acetone sensors. The enhanced gas sensing performances can be attributed to the factors like ultra-uniform 3D nanotube-like structure for alpha-Fe2O3, the noticeable advantages of PANI, as well as the construction of heterojunction barrier at their interface. The as-prepared PANI/alpha-Fe2O3 nanocomposite may be also applied in other areas like photocatalysis, water purification, ion batteries, optical devices, and so on.