10,12-Pentacosadiynoic acid(DA)-assembled on the surface of α-FeOOH was irradiated with ultraviolet light (=254 nm) to obtain polydiacetylene/α-FeOOH(PDA/α-FeOOH) nanocomposites. The structure and morphology of nanocomposites were analyzed by XRD, SEM and DLS. Thermochromism experiment showed that PDA/α-FeOOH had a reversible change from blue to red between 30℃ and 80℃. The effects of irradiation time and amount of α-FeOOH on reversible thermochromic of PDA/α-FeOOH nanocomposites were investigated. The results revealed that bule PDA/α-FeOOH nanocomposites(m(DA):m(α-FeOOH)=3:1) with UV light irradiated for 10 min was the fastest color change. When the composites were warmed at 60℃, its color changed red and the maximum absorption wavelength happened shift-blue from 650 nm to 540 nm. The heating and cooling cycles indicated that PDA/α-FeOOH composites had higher thermal stability.