A droplet micro-reactor was developed to realize the internal sol-gel reaction of ZrO2 inside the droplet. The reactants are only allowed to combine when the droplet forms, meanwhile, they are able to mix well and react to form gel particles with a proper designed structure of the micro-reactor. In this condition, the mixing efficiency was determined by the structure of the micro-reactor and the flow rate ratio of the two components. After sintering, ZrO2 ceramic microspheres are acquired with high sphericity and narrow size distribution. This droplet micro-reactor overcomes the gelation of the premixed sol and reactant in the channel, and it has been demonstrated to be more stable with a longer running time.