Droplet microfluidics generally employed syringe pumps to infuse fluids into microchannels that created unavoidable interface fluctuation during emulsification. They also caused droplet differentiation among different batches due to the reloading of syringes. We herein reported a new gravity-driven approach to supply a steady pressure input on the microchannels and continuous liquid supplement due to the semi-open design. This microfluidic system combined by several overflow units could effectively reduce the disturbance on the multiphase interface, and single emulsions with high monodispersity could be continuously generated. By changing the overflow height, different sizes of single emulsions could be obtained. In addition, double emulsions could also be realized by simply adding another unit without effecting further changes in the system. These features indicated that the gravity-driven overflow microfluidic system could infuse fluids steadily and continuously, and could widely contribute to fields requiring less manual power.