Abstract: No correspondence of processing results in both LS and difference methods is always difficulty to explain clearly in practical. In this paper, first prove that k order difference has the same frequency response as (k 1) order polynomial smoothing of (k+1) data, which reveals the inner connection of two methods. Second, produce the computation formula of correlation interval. Finally, present the new method of separating random error from measure data. Theory and practical processing results show that the new method has high precision.