With foamed concrete as the absorbing substrate, under rapid water loss, the hydration mechanism of cellulose ether modified thin layer cement pastes was studied. The cement pastes were divided into three layers from surface to interface. Each layer cement paste of 6 h, 12 h, 1 d and 3 d ages was analyzed by using water loss speed, moisture content, XRD, FTIR and TG DSC DTG. The results show that cement pastes of different layers quickly lose water when hydration time is less than 6 hours and characteristic diffraction peaks of Ca(OH)2 are detected only in the second and third layer. The water loss speed significantly decreases when hydration time is more than 12 h and characteristic diffraction peaks of Ca(OH)2 are detected in every layer. The content of Ca(OH)2 increases continuously from the first layer cement paste to the third layer cement paste and aggregation state of ［SiO4］4- in C S H gel changes. The intensity of diffraction and vibration peaks of Ca(OH)2 within cement pastes increases as hydration time prolongs.