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In the ultrasonic detect processing of optical fiber fusion defect, because of the noise pollution some weak bounding defect is difficult to detect exactly. This paper proposed a modified threshold function based on the wavelet multiresolution analysis threshold de-noising method, and applied this method on defect signal de-noising. This threshold function overcomes the discontinuous shortcoming of the hard-threshold function and the disadvantage of soft threshold function which is the invariable dispersion between the estimated wavelet coefficients and the decomposed wavelet coefficients, which has many advantages, such as sample expression and convenient calculation. The actual test results of defect noise signal show that this method is able to get the lesser mean-squared error and improve the signal-to-noise ratio of reconstruction signals through comparing this modified threshold function threshold de-noising algorithm with the methods of hard-threshold and soft-threshold, therefore which have excellent de-noising results.