We present a systematical experimental study on droplet deformation under alternating current (AC) electric field in a microfluidic channel. Effects of conductivities and surface tensions on droplet deformation are respectively investigated. It is found that for droplets with low conductivity, droplet deformation depends on both the applied electric field strength and AC frequency. When electric force dominates, droplet deformation decreases with the increasing AC frequency and behaves like a low pass filter. For droplets with higher conductivity, the effects of AC frequency disappear, and droplet deformation stays in the same level at given electric field strength. An equivalent electric circuit model is proposed to explain the frequency dependence of droplet deformation. A force analysis derived from Maxwell stress tensor shows that for droplets of low conductivity, the electric force mainly results from the electric permittivity force, which has distinct magnitudes at different AC frequencies, but for droplets of higher conductivity, the charge force contributes most to the electric charge force, and it depends on the product of the droplet conductivity and the effective elelctric field strength exerted on the droplet. Finally, We categorize a modified electric capillary number to explain the effects of surface tension on droplet deformation under electric field, and the experiment results have good agreement with the theory.