Tune is the essential music component. Melody extraction is a crucial technique in music retrieval. The pitching order of polyphonic music is the main feature of vocal music. This paper presents an improved method of extracting vocal themes based on the quadratic wave equation. This paper presents a new algorithm based on the frequency spectrum characteristics of verbal signals. This method can effectively reduce the amount of computation and shorten the time of the primary melody extraction. This paper proposes a new fundamental frequency recognition model based on BP neural network, which has a good ability for secondary wave transformation. This method can improve the recognition accuracy of the primary melody model while reducing the error of melody location. This can improve the overall vocal music of the leading melody extraction precision. The analysis results of MIR-1K data show that the overall recognition accuracy of the improved theme is 2.13% higher than that of the original method. Compared with the traditional way, the average time of theme extraction is shortened by 0.06 seconds.