The characteristics of cockpit voices (sounds) recorded by Cockpit Voice Recorder (CVR) are key evidences in investigating accident causes for wrecked airplane. In order to analyzing and diagnosing wrecked airplane causes through cockpit voices in CVR, some researchers are made as followings in this paper: Firstly, some typical background sounds of cockpit voices, such as wind shear audio, near-earth audio warning, take-off form of audio warning, fire alarm and so on, are obtained and classified through listening and distinguishing by adobe audition audio software in laboratory. Then, the characteristics of these background sounds are extracted by signal analyses methods such as Fourier Transform (FT), Wavelet Transform (WT), and so on. Through these methods, the special characteristics are depurated, such as frequency value, spectral density, and frequency line numbers. Thirdly, as the key part of the paper, conditional rules and fault trees principles focuses on and applied in order to distinguish and diagnose these special characteristics for approximate or different background sounds of cockpit voices. And some available results are obtained finally. Through all these above researches, new analyzing and diagnosing approaches are put forward, which are suitable for accurate grasping the cause of flight accident and analyses and diagnose flight accident. All the researches and conclusions have a certain reference for analysis and diagnosis of flight accidents, and protect flight security.