A method to improve the gain of axial-mode helical antenna is proposed. This method involves a parasitical circular metal disk,which is installed on the front of general axial-mode helical antenna and is apart from the helical line. A circular current whose phase lags behind that of helical line current appears,which brings a more concentrated radiation field. Consequently,the antenna gain is improved, Based on the simulation results,an antenna array model fed independently is proposed.This model gives an excellent explanation of the radiation characteristic of helical antenna, Both the simulation and experiment results show that for obtaining the same gain,the antenna length in this new method is only 71% of that in traditional helical antenna.The reduction of antenna length favors the miniaturisation of antenna. In addition, this method has a little effect on the bandwidth of antenna, so it can be widely used in the design of helical antenna element and array.