In order to obtain excellent tomographic image of samples, a scanning magnetic field with good linearity, long scanning time and large amplitude must be used in an electron spin resonance imaging system. In this paper a method has been presented, in which a combination of software and hardware is applied and good experimental results have been obtained. That the magnetic field scanning and collection of data are unitedly controlled by computer to make them synchronized, and that in collection of data first the data is added repeatedly then is averaged over them to make the noise reduced remarkably are also introduced.