In order to solve the problem that it is difficult to extract the watermark when the carried audio is subjected to malicious attacks, audio watermarking algorithm based on spectrum distribution is proposed. An eigenvalue is designed to represent the spectrum distribution of the specified frequency-band, and then the binary watermark is embedded by adjusting the difference of the eigenvalues between two adjacent frequency-bands. The polarity of embedding depth is determined by comparing the values of the eigenvalues, which will minimize the modification about the audio so as to improve transparency. The binary watermark can be extracted blindly by judging the difference of the eigenvalues. The improved synchronization mechanism takes the small frame with the largest energy in the voiced frame as the synchronization mark to search the embedding and extracting location of the watermark, which makes the algorithm have strong robustness. Experimental results show that the proposed algorithm has large payload capacity, good transparency, low complexity, and strong robustness against most attacks.