Effect of decalcification on microstructure of CSH gel in ASTMⅠcement paste were studied by Xray fluorescence spectrometer(XRF), Xray diffraction(XRD), infrared spectroscopy with Fourier transform(FTIR) and 29Si MAS NMR with deconvolution technique. It is shown from test results that the decalcification process can be divided into two stages. When the molar ratio of CaO to SiO2 in the pastes at 3, 28d is lowered from~2.78 to~2.00, almost all of CH is removed and CSH gel is partially decalcified, the average chain length(ACL) of CSH is increased from 2.4 to 4.4 for 3d paste and from 2.9 to 5.8 for 28d paste respectively, and I(Q2)/I(Q1) is also increased. While the molar ratio of CaO to SiO2 in the pastes comes to~1.83, decalcification of CSH gel occurs. CSH polymerizes and I(Q2)/I(Q1) increases largely in this stage, which results in further increase of ACL to 6.2 for 3d paste and 9.9 for 28d paste. The mechanism of increase of CSH polymerization could be transformation of dimeric CSH toward highly polymerized CSH. However, Al doping could affect the stability of CSH microstructure. In molecular level, the existence of the pure bridging siliconoxy tetrahedrons in CSH is beneficial to the optimization of the microstructure.