Sulfonated acetone–formaldehyde polycondensate(SAF) was fractioned by ultrafitration into four fractions with molecular weight range of ＜2500、2500~10000、10000~50000 and ＞50000. Structural analysis showed the weight-average molecular weight and intrinsic viscosity of the four fractions successively increased while their sulfonation degree decreased. Meanwhile, properties of SAF with different molecular weight as water reducer of cement were studied. Results showed with the increasing of molecular weight of SAF, the water-reducing ratio of cement paste containing SAF increased remarkably, but the paste tended to lose fluidity easier with time-lapse. And the retardation effect of SAF decreased because the content of sulfonic groups and hydroxyl reduced and the complexation of these two groups to calcium ion was weakened. The 7d and 28d compressive strenght of harden mortar added fracioned SAF were enhanced by 30%~60% in contrast with the reference and the strenght increased as molecular weight of SAF increased. The two fractions of higher molecular weight had dosage and water-amount sensitivity ,and when their dosage was above 0.3%, mortar became bleeding. It thus decreased the compressive strenght.