In this paper, the characteristics of the two-dimensional hole gas (2DHG) in p-GaN/p-AlxGa1-xN heterojunction is investigated in detail, based on self-consistent solutions of one-dimensional Poisson and Schrdinger equations. The valence band structures and the 2DHG distributions are calculated in the cases of different Al components and piezoelectric polarization effects. Then, the influences of Al components and piezoelectric polarization effects on 2DHG are analysed specifically. The results show that with the increase of Al component, the quantum well at the heterojunction interface turns deeper and narrower, which leads to an accelerated growth of the 2DHG peak density and a line increase of the 2DHG sheet concentration. Furthermore, piezoelectric polarization effects also make the quantum well at the heterojunction interface deeper and narrower, at the same time, the Fermi level moves close to the top of the barrier and the location of peak density moves close to the heterojunction interface. In addition, the influences of valence band offset and acceptor doping concentration on 2DHG are relatively small. Ohmic contact of p-AlxGa1-xN is fabricatea with the 2DHG, and its I-V characteristic is much better than that without the 2DHG, which indicates that the 2DHG can significantly improve the performance of p-AlxGa1-xN ohmic contact.