Abstract: It is supposed that the observations obey a certain distribution in all the methods of parameter estimation to this day. But in the practical surveying, due to the different observation conditions, the observations obtained under different observation conditions may obey different distributions. So the supposition is not conform to reality. The idea that conform to reality should be that the distribution of the observation is estimated based on the observations before estimated parameter. According to the idea, this paper introduces the theory of information spread at first. Then the method of information spread estimation (ISE) has been presented. The theory and calcultion indicate that ISE not only has fine robustness, but also it is very simple and convenient for application.