A new planar differential helix antenna for 5G millimeter-wave applications is proposed in this paper. The proposed antenna has a simple rotational symmetrical planar structure fed by strip line, which is printed on one substrate. The width of the helix are gradually increasing to improve the RHCP gain. A step ground is used to obtain a better impedance matching. Furthermore, to enhance the vertical polarization and thus improve CP gain, parasitic unit directors formed by metallized via-holes and printed stripes, are introduced. The simulated results show that the -10-dB impedance bandwidth ranges from 24GHz to 30.9GHz (24.6%), and the 3-dB axial ratio bandwidth is from 23.8GHz to 29.1GHz (20%). The proposed antenna features simple structure, low cost and good performance, thus it has a good application prospect in 5G millimeter-wave applications.