In order to study the evolution law of calcium silicate hydrate gel(C S H) average chain length during the carbonation process, the cement paste specimens prepared from different water cement ratios (mw/mc) and carbonation ages were studied by using the nuclear magnetic resonance(NMR) spectroscopy. Meanwhile, the content of calcium hydroxide(CH) and CaCO3 was calculated by thermogravimetric experiment. The experimental results show that the C S H average chain length changes from low to high under the condition of accelerated carbonation. For the same mw/mc specimen, its C S H average chain length increases with the extension of carbonation age. After 28 days carbonation, 053mw/mc specimens C S H is almost completely carbonated and 035mw/mc specimens C S H structure changes to certain extent, while 023mw/mc specimen changes barely. In addition, the carbonation of 053mw/mc specimens C S H does not begin with CH completely reacted; after 7 days of carbonation both CH and C S H undergoes carbonation reaction but the carbonation is not completed.