The separation , purification and concentration of the polysaccharides from Opuntia monαcαnthαHaw.were carried out via membrane ultrafitration using the membranes with different cut-off molecular masses. The effects of several parameters such as the pressure , the temperature , the purification time , the flow rate and the con-centration multiple on the ultrafiltration process were then investigated. The optimal conditions for the ultrafiltration were finally determined. It is found that , for the membrane with a cut-off molecular mass of 10 000 ku , when the pressure , the temperature , the flow rate and the concentration multiple are respectively0. 20 MPa , 25℃ , 0. 04-0.05 Umin and 5 -7 , a yield of crude polysaccharides of 0.95%can be reached. Moreover , as compared with the traditional vacuum concentration , membrane ultrafiltration is easier to obtain high-quality crude polysaccharides with a higher yield , which is helpful to the further extraction and purification of polysaccharides in the subsequent process.