Abstract: Using the morphological filters for de-noising, the determination of the structuring element's size is the most important step. However, most of the determination methods depend on either the experience or a comparison with the ideal waveform for optimizing structuring element's size, the former is lack of accuracy, the latter is lack of practical significance. Aiming at these problems, the paper proposes a new method that uses a detection factor function determinate the structuring element's size.Firstly, the message of signal structuring size was obtained by the use of detection factor function.Secondly, with the derivation of the message, proper sizes of the structuring elements were acquired.Finally, the multiscale pyramid type filters was used to de-noise the signal. Through the simulation of random signal de-noising and the de-noising experiment of lidar return signals, this method demonstrated better de-noising effect. Comparing with other usual morphological filtering methods, it neither depends on experience nor the introduction of the ideal waveform. Therefore, this method has a promising future.