Abstract: By means of FT-IR and subtraction techniques the absorption spectra of Poly A film have been studied systematically in the 4000 to 800 cm~1 region at (25±1) ℃ as a function of relative humidity (from 0％ to 100％) at static equilibrium. According to the peak position and intensity changes of infrared bands it proves that the hydration of Poly A has happened mainly on the-NH_2 group of pbase, -PO_2 group and ribose ring; and the possible hydrating sequence is as shown in Fig.9. In comparision with the previous investigators~[2] many new informations are obtained. First, the —NH_2 band (～1712 cm~(-1)) shows strongest variation in intensity at RH≤30% (Fig.4) and obvious frequency shift at RH≥75% (Fig.6). So it is one of the strongest hydrating sites. Then, the P=0 band (～1240 cm~(-1)) does not only behave as a 20 cm~(-1) frequency shift pointed out by Falk but splits into two peaks and coexists at high relative humidity (see Fig.3 and Fig.5). Besides, the spectra of water molecule also varies obviously and splits into multiple peaks during hydration (Fig.7). This indicates that the molecules bonding to Poly A have different structure status.