Microfluidic fabrication of helical microfibers is still a big challenge. The reason is that this always includes designing the necessary geometrical channels and chemical conditions to first form a flowing liquid jet, which has to be continually reacting and rapidly evolving in time from viscous liquid to a flexible solid to maintain the helical structure inside the microfluidic channels. In this report, dextran aqueous solution and liquid PEG400 are infused separately into the inner and outer channels of a simple single emulsion microfluidic device, respectively. The formed two phase stream then enters a widening collection tube, where automatically formation of dextran helical fiber happened due to water shifting and widening of the channel cooperatively induced buckling. Various experimental conditions that influence the amplitudes, wavelengths, and diameters of the formed helical fibers are discussed.