The theory and simulation technique of weak signal detection using chaos state phase change is researched in this paper. The method of weak signal detection by using the Duffing oscillator's initial conditions sensitivity was firstly analyzed， the existing transition process would affect the detection capability. Then an improved method was presented. In order to compare these two methods， the input pseudo-noise expression and the calculating step selection in simulation were researched， and the simulation models were established. The new method was shown to have better capability through analyzing the signal detection experimental results under the condition of typical noise intensity background. The experiments showed that the Duffing oscillator in special critical chaos state is also sensitive to noise， which leads to the difficulty in exactly deciding the minimal detection limit by phase change， and the limitation of the phase change method was discussed.