Abstract: The technology of composite targets is applied in many mid- and high-energy band medical accelerators to harden the energy spectrum of X rays for radiotherapy treatment. Primary studies of composite targets by EGS4 code are presented. The dose distributions of electrons and X ray are given by simulation computation. After using composite targets that are made up of 2 mm tungsten and a filter of 3 mm copper or 10 mm graphite, in contrast to simple targets, the dose of electrons reduce to 10%, the dose of X rays keeps optimal, and the average energy of X rays rises. Composite targets can be used to harden X rays and reduce electron pollution.