Abstract: Tianma telescope is a real surface altazimuth radio telescope with a diameter of 65m. Its post-fed, low-temperature and low-noise receiving system collects the signal collimated by the shaped parabolic main-reflector and the double curved sub-reflector. This paper studies the axial defocus effect of Tianma so as to improve its receptivity at low frequencies. A P-band dipole antenna, placed 1/4 wavelength beneath the sub-reflector, is used as the feed of the receiver to explore the performance of Tianma telescope at low frequencies. This paper is divided into three parts:the design of the P-band dipole antenna, the optimization of the axial feed defocusing position and analysis of its observed performance. Results suggest that the maximum gain of the dipole antenna is 45dB, with a high efficiency of 64.25%.