Effects of citric acid on crystalline habit and crystal morphology of gypsum dihydrate were analyzed in great detail. The retarding mechanism of citric acid was also discussed from the standpoint of crystal growth by use of Scanning Electric Microscope and X-ray Photo Technology. The results shows that due to the complexion between citric acid and Ca2+, the initial supersaturation degree in liquid phase is greatly decreased, correspondingly the number of nuclei newly formed reduces and the growth rate of dihydrate slows down, so the crystal has enough time and space to grow, as a result, the crystal size becomes much larger in presence of citric acid. Citric acid is preferably adsorbed on c-axis which has the fastest growth rate and hinders its growth. Therefore the relative growth rate and crystal habit of various crystal faces are changed, consequently leading to the transformation of dihydrate crystal from need-like to short prismatic. Furthermore, the transformation is gradual as a function of citric acid addition, the higher the dosage, the larger the dihydrate crystal formed. After the addition of citric acid, the retarding effect produced and the reduction in strength of hardened gypsum paste are closely related with the variation of crystallization habit and crystal structure of hydration products.