In this paper， we adopt the thoughts and methodologies of Shannon’s information theory and use the distance-amplitude and phase mutual information between the received signal and the measured object as a measure to propose a unified definition and description method of the target distance information and amplitude-phase information and derive their closed expression. Thus， we establish an information theory model for multi-carrier radar systems. Studies have shown that target distance information and amplitude-phase information will interact with each other， and amplitude-phase information usually depends on distance information. Besides， the amplitude-phase information of the target is only related to the SNR but not to the distance information when the scattering coefficient of the target obeys the complex Gaussian distribution. This paper further studies the relationship between the distance information of the target and the maximum likelihood estimation. Theoretical analysis and numerical simulation results show that the Cramer-Rao bound is an asymptotic upper bound for distance information at high SNR. However， the actual radar system usually works in the medium SNR environment， and its theoretical boundary is not clear for a long time. Now， the distance information of the target can be used as the upper bound of the maximum likelihood estimation， which provides a basis for the performance of the actual system. This conclusion reveals that radar information theory has an important guiding role for practical applications.