Abstract: Digital signal automatic identification  based on extracted instantaneous feature has important applications.While adopting the method to identify signal,we must  extract the instantaneous amplitude,phase and frequency of the signal to calculate the classification parameters.But phase wrapping occurs when extracting instantaneous phase.This paper analyzes  phase wrapping and unwrapping in detail,then  proposes a method to estimate the theoretical threshold range of phase unwrapping.And other relevant references didn’t carry out an indepth study on the threshold.Simulation results of matlab show that nonlinear instantaneous phase can be effectively and directly extracted if adopting the phase unwrapping threshold range estimated in this paper.