The performance of the Page Test decreases without the exact knowledge of the transient signal length.A new method independent of the signal length is proposed in this paper.The detector of unmatched signal length of the Page Test is modeled at first.Then the theoretical detection performance and the simulative one of the new method are compared to the general Page Test while detecting the unknown length transient signal.The experiment data on the sea of certain bearing channel after beamforming are exploited to compare these two methods.The result shows that the performance of the method proposed in this paper is more stable than the general Page Test without the exact knowledge of the transient signal length.