We theoretically analyze the electrical properties of GaAs substrate-nanowire p-n junction using Technology Computer Aided Design (TCAD). The results show that GaAs nanowire-substrate p-n junction exhibits clear diode behavior which has been confirmed in some experiments. Increasing NWs doping concentration and diameter or shortening NWs length can enlarge the total current after the diode is conducted. Total current through p-n junction is approximately linearly proportional to NWs growth density. The substrate-nanowires p-n junction has smaller current than that of substrate-layer p-n junction, which implies the superiority of NW structure. These results constitute an important progress for experimental researches on nanowire-integrated devices.