The moisture absorption characteristics and the changes in the static and dynamic mechanical properties of the pultruded unidirectional carbon fiber/vinyl ester resin (CF/VE) composite, which is used for making sucker rods for oil fields, were studied during the immersion of it in a 5% H₂SO₄ aqueous solution at 65℃ and 95℃. The results showed that at the early immersion stage in the solution, the moisture absorption behavior of the composite was similar to the Fick's second law, that is, the moisture content of the composite increased rapidly at the early immersion and was linear with t^1/2, but the moisture content decreased a little at the late stage. The storage modulus (E′) of the pultruded CF/VE composite decreased with the immersion time, while the loss factor (tanδ) increased. Furthermore, the higher the temperature was the more serious the effects were. The decreases in the glass transition temperature (T_g), flexural strength and the interlaminar shear strength of the composite were relative to the increase in the moisture content.