High modulus and thermal stability of polyimide (PI)/reactive halloysite nanotubes (HNTs) nanocomposites were prepared by in situ polymerization. The pristine HNTs were firstly handled with the tetraethoxysilane (TEOS) and secondly grafted with the silane agent. The fourier transform infrared spectroscopy (FTIR) approved that TEOS was beneficial for the silane agent to modify the HNTs. Scanning electron microscopy (SEM) showed the differences of the morphology between the reactive HNTs and pristine HNTs. PI/reactive HNTs nanocomposites exhibited lower moisture absorption than pure polyimide. The reactive HNTs reduced the transmittance of the nanocomposites. Significant improvements in the thermal stability and glass transition temperature (Tg) of PI/reactive HNTs nanocomposites were achieved by addition of only a small amount of reactive HNTs. It was noteworthy that both the tensile strength and Young' modulus of PI/reactive HNTs nanocomposites were significantly enhanced. A 62.8 % increase in tensile strength and a 63.7 % increase in Young' modulus of the nanocomposites with 3 wt.% of the reactive HNTs were achieved. Finally, the preparation mechanism to obtain PI/reactive HNTs nanocomposites was proposed.