A theoretical model of target detection is established for a general radar system. Specifically, we employ the thoughts and methodologies of Shannon's information theory to research the location information and amplitude-phase information of targets in a radar detection system. In the proposed model, we derive closed expressions of location information and amplitude-phase information in a single target detection assuming the target follows a uniform distribution in the observation range for a constant scattering coefficient and Rayleigh distributed scattering coefficient correspondingly. And in the high signal-to-noise(SNR) regime, we obtain an analytic expression and Cramér-Rao Bound(CRB) of location information for a constant scattering coefficient. Theoretical analysis shows that the location information of the target is linearly proportional to both time-bandwidth product(TBP) of radar detection system and the SNR in dB. The simulation results validate the theoretical analysis and give two important stages of radar target detection, namely, target acquisition and target tracking. The work of this paper is of great theoretical significance for the design of real radar detection systems.