Calcium alginate (CaAlg) hollow microfibers were continuously and controllably prepared by microfluidic with oil phase as isolation layer. The inner and outer phase fluids were extruded through the outlet of the microfluidic device, and the coaxial jet was formed in the oil phase isolation layer by adjusting the distance between the outlet of the device and the solidification bath surface, the flow rate of the outer phase and the collection speed. The CaAlg hollow microfibers were formed after the jet templates cross-linked in the coagulation bath. In the experiment, the optimal distance between the outlet of the microfluidic device and the surface of the coagulation bath is 2 mm. The larger the flow rate of the outer phase and the smaller the collection speed, the more stable the jet. Within the range of conditions for the formation of a stable jet, CaAlg hollow microfibers with uniform structures were prepared. The diameter of the hollow microfiber can be precisely controlled by adjusting the collection speed and flow rate of the inner and outer phases. A simple analysis model was established according to the flow continuity equation to predict the diameter of hollow fibers. The model gives an explicit relationship between the inner phase flow rate, outer phase flow rate and collection speed with the microfiber diameter. The accuracy of the model""s prediction was verified based on the experimental data.