To determine the hygrothermal properties of natural bamboo fiber(BF) as a building filler material and the hygric thermal interaction rule, associated studies were carried out, including test items for equilibrium moisture content(u value), moisture absorption and desorption rate(U value) and thermal conductivity(λ value), and calculation items for thermal storage coefficient(S value) and index of thermal inertia(D value). Results show that there are exponential relation between u φ, U φ and λ φ that can generally be expressed in the form Y=a·eb·φ+c; within the range of the test, the u φ curve and U φ curve grow rapidly when φ is 854%963% and φ is 75%95% respectively; the dry λ values of BF with bulk density ρ between 70170kg/m3 are 0042 30046 5 W/(m·K); the wet λ values increase significantly with the rise of φ, and the growth rate is in positive correlation with the ρ values. The increase of u value and the decrease of U value can improve the thermal performance. In particular, increasing the u value can raise the S value and D value of BF, and reducing the U value can significantly improve the thermal stability of BF.