The characteristic of X ray source is one of the most important parameters on the image quality of radiography. The X ray source is composed of bremsstrahlung photons produced by the high energy electron beam impinging on the heavy metal target. The effects of parameters (include radius and emittance of the e beam and the target thickness) on the characteristic of X ray source are detailed by means of Monte Carlo method. It is shown that (1) giving a e beam radius, the exposure at 1m from the converting target increases and the uniformity of exposure distribution becomes bad, when the normalized emittance of e beam decreases; (2) for a given normalized emittance, the exposure uniformity becomes bad, when the e beam radius increases, but there exists an e beam radius under which the exposure at 1m is the largest; (3) in the case of zero emittance, enlarging the e beam radius can slightly improve the uniformity; and (4) given an e beam, there exists a target thickness which causes the exposure at 1m