Abstract: The ultra-low permeability reservoirs in Changqing Oilfield are characterized by tight structure, poor physical properties, and high heterogeneity. The broadband fracturing technology adopted has promoted the development effectiveness and fulfilled the objective of increasing oil production and reducing water cut. However, current studies rarely focus on building a productivity prediction model, and the water flooding mechanism remains to be clearly perceived. Therefore, this paper was designed to accurately evaluate the stimulation effect of broadband fracturing in Changqing Oilfield and further reveal the seepage rules. The physical seepage field around the water injection well was divided into three zones according to the flow pattern of the oil-water two-phase flow, namely the main fracture zone with a linear flow, the broadband zone with an elliptical flow, and the matrix zone with a circular radial flow. Furthermore, the equivalent flow resistance method was employed to build a productivity model for broadband fracturing of the diamond inverted nine-spot well pattern with non-Darcy seepage. With an actual well group as an example, the influence brought by broadband width and equivalent permeability on the productivity of the well pattern were analyzed by numerical computation, and the change rules of daily liquid production were explored. This results suggest that the broadband fracturing technology dramatically boosts the daily oil production, recovery degree and daily liquid production of the well pattern, enlarges the lateral swept volume, enhances the uniformity of water flooding, and improves the productivity. The results of this paper can provide theoretical support for oilfields to optimize the technology of broadband fracturing on site.