Bremsstrahlung converter target is one of the key factors for flash X-ray machine. The bremsstrahlung radiation process in which the electron beam impinges on the target is simulated utilizing the Monte Carlo method, by which the influence of the effective target thickness on the exposure is analyzed, the optimal target thickness and the exposure are also obtained for the 4 MeV flash X-ray machine. The damages of target impacted by the electron beam are compared and discussed for various electron energy deposit density from different sources, such as 12 MeV LIA, Dragon accelerators and 4 MeV flash X-ray machine. The results show that the tantalum distributed target may be the satisfactory solution for the bremsstrahlung target of 4 MeV flash X-ray machine.