Harvesting efficiency is one of the key issues restricting the further commercialization of microalgae. Traditional biofilm studies only tend to focus on enhancing biofilm formation, while neglecting the subsequent harvesting of the biofilm. In this study, a win-win strategy for high-density microalgal cultivation and low-cost harvesting through biofilm cultivation on a pH-responsive, charge-switchable, patterned membrane was investigated using a polyethylenimine (PEI)-crosslinked polyvinylidene fluoride (PVDF) membrane. The patterned membranes were prepared by spray-modified non-solvent induced phase inversion, and the effect of pattern height on biofilm formation and microalgal harvesting was investigated. The PEI-crosslinked PVDF membrane surface was positively charged below pH 8 and negatively charged above pH 9, resulting in a higher attractive energy below pH 8 and a repulsive energy above pH 9 according to the extended Derjaguin, Landau, Verwey, Overbeek model. Patterning of these membranes increased the active area for microalgal attachment, and created a low-shear area in the valleys that prevents early detachment of microalgal cells. Membranes with a higher pattern height resulted in faster biofilm development and increased final biomass accumulation. Low-energy membrane vibration was applied to enhance cell detachment. pH=7 was found to be the optimal pH for enhanced microalgal biofilm formation on PEI-crosslinked patterned membranes, and a switch to pH=10 was best for microalgal harvesting.