Most porous materials with high specific surface area and diverse internal structures possess good adsorption ability. In this work, a tremella-like mesoporous calcium silicate hydrate (CSH) with high adsorption capacity was successfully prepared via a facile hydrothermal method. The adsorption effect and adsorption mechanism of the as-prepared calcium silicate hydrate (AP-CSH) towards formaldehyde from water were investigated systematically. Results indicate that AP-CSH has high Ca/Si ratio (1.95), large specific surface area (122.83 m2 g−1) and exhibits excellent adsorption capacity. The results of batch adsorption experiments show that AP-CSH can remove formaldehyde from water rapidly and effectively with the maximum removal efficiency of 98.94%. The adsorption process agrees well with the pseudo-second-order and Freundlich isotherm model. Furthermore, regeneration can be achieved by simply immersing AP-CSH in absolute ethanol and the removal efficiency can still reach about 99.50% after five cycles. The adsorption mechanism was also studied by experimental analyses and molecular dynamics simulation. Both experimental results and theoretical simulation support that formaldehyde adsorption over AP-CSH belongs to chemical adsorption.