Alginate-based hydrogels are attracted much attention in biomedical and chemical field, and their size and shape are significant to their applications like drug delivery and cell encapsulation. Monodisperse sodium alginate microdroplets are produced using a flow-focusing microfluidic device (MFFD) by adjusting the flow rate on the continuous phase (soybean oil) and the dispersed phase (sodium alginate solution). The external gelation process of sodium alginate microdroplets occurs outside the chanel in a calcium chloride solution to form calcium alginate hydrogel particales. The shape, size and size distribution of these calcium alginate hydrogel particles depend strongly on the flow rate inside the MFFD. By optimizing the parameters, the hydrogel microparticles were obtained with diameters ranging from 70 µm to 100 µm with size distribution under 10%, depending on experimental conditions. The removal of Cu2+ ions by the absorption of hydrogel microparticles was also demonstrated.