The Geant4 code was used to model components of the head of Siemens medical linear accelerator with the field commonly used. Then we simulated the process of electron beams, which could produce 6 MV X-rays, bombarding a gold target. By comparison with the measured data, we ensured that the model was built correctly. Then we obtained the information of all the particles to the field plane, and wrote it to a phase space file. The kinds of particles and their average energy, energy spectrum distributions, particle fluence distributions, energy fluence distributions, and angular distributions were presented by processing the file with the procedure written by myself. The results show that the average energy of photon is higher than that of electron and positron; energy spectra of photon and electron show continuous distributions; electronic particle fluence is two orders of magnitude less than that of photons; particle fluence and energy fluence of photons are evenly distributed in the field and decrease rapidly out of the field, while the electronic’s fluctuate considerably in the field and smoothly out of the field. Angular distribution of photon is mainly concentrated within the range of 10° off the central axis, while the electronic’s are in a relatively larger range.Cited