Based on the phase field method (PFM) and charge conservation equation, a numerical method is proposed for two-phase flows in an external electric field within the OpenFOAM framework. Under weakly conducting condition, the deformation of a single droplet and the coalescence of two droplets under alternating electric field are investigated. The results show that the frequency of the alternating electric field can effectively affect both the deformation rate of the single droplet and the coalescence efficiency of the two droplets. When the permittivity ratio Q equals to the electrical conductivity ratio R, the mean deformation resulting from the alternating electric field (AC field) is the same as the steady state deformation under the equivalent direct current electric field (DC field), and the coalescence time becomes longer with the increase of the AC field frequency. When Q＞R, the mean deformation rate of the single droplet increases continuously and the coalescence time of two droplets reduces with the increase of the frequency of the AC field. When Q＜R, the increase of AC field frequency leads to the decrease of the average deformation rate of single droplet and the increase of the fusion time of two droplets.