Response surface methodology was applied to optimize the technological parameters of the ultrasonic-assisted alkaline extraction of soluble dietary fiber from dried osmund. Effects of four independent variables including solid-liquid ratio, extraction temperature, ultrasonic time, and ultrasonic power on the yield of soluble dietary fiber were studied by single-factor experiments. A quadratic regression mathematical model was established by Central-Composite experimental design and analyzed by response surface methodology. Meanwhile, the influences of independent variables and their interactions on the response value were discussed. The optimal technological parameters were determined as follows:alkaline concentration 0.02g/mL,extraction time 90min,solid-liquid ratio 1.0g/23mL, extraction temperature 54℃, ultrasonic time 25min, and ultrasonic power 380W. Under the optimal extraction conditions, the yield of soluble dietary fiber was up to 38.39%.