Spatial information theory is the fundamental theory of radar and other target detection systems. The spatial information is associated with the range, the direction and scattering signal of targets. Parameter estimation theorem is an important part of the spatial information theory. The proof framework of the parameter estimation theorem is proposed in this paper, which consists of three original concepts of spatial information, entropy error and sampling posterior probability estimation, and one theorem. The parameter estimation theorem shows that the entropy error is reachable, otherwise the empirical entropy error of any estimator is no less than the entropy error. The establishment of spatial information theory will play a significant role in promoting the development of systematic theories and methods of radar.