Abstract: The forward skewed rotor blade of low speed axial flow fan can increase the efficiency by 4～5%,expand the range of off design condition by 15～20% in the comparison with the typical blade straight blade.In this paper,the “di actuator disc”theory model is applied to calculating the flow field of forward skewed rotor blade;the modified loss model in design condition is adapted to the lift/draft model of attack angle and is used to estimate the off design condition flow field,the efficiency of the blade,etc.The estimated results at the design condition are in good agreement with the experimental results [1] .The maximum error of the efficiency along the blade height is 1.5% and that of the total pressure is 1.8%.The mass average errors are 1% and 1.5%.At off design condition,the mass average errors of the efficiency and the total pressure are all less than 2%.It is proved that this method is applicable and can meet the need of the engineering design.