The electrokinetic motion of a negatively charged oil droplet attached to the negatively charged water-air interface from below is numerically studied for the first time by a three-dimensional numerical model in this paper. The effects of the mobile water-air interface and the mobile water-oil interface on the electrokinetic motion of the attached oil droplet are investigated and discussed in terms of the zeta potentials at the water-air interface and the oil droplet surface, the applied electric field, the dynamic viscosity ratio of oil to water, and the droplet radius. The results show that the negatively charged oil droplet attached to the negatively charged water-air interface from below moves in the opposite direction to that of the external electric field, and its moving velocity increases with the increase of the electric field strength, the magnitudes of the zeta potentials at both the water-air interface and the water-oil interface, and the droplet size, as well as the dynamic viscosity ratio of oil to water.