The acoustic pressure signal near the engine valve cover contains many useful information which can diagnose the fault of diesel engine, these information includes combustion, the valve open and close information. But this signal also contains many interfere noise, like exhaust noise and other noise eradiated from the other part of the diesel engine, and the frequency composition of useful signal and interfere noise are largely overlapped in the frequency band under 20KHz. So how to abstract the useful signal is very important. In this paper,1CM outside the valve side of the engine valve cover was selected as the measure point. Four kinds of signal processing methods: wavelet decomposition, FIR filter, wavelet denoise and FIR divided frequency bands threshold valve denoise was investigated . We find wavelet denoise can reserve useful signal and eliminate interfere noise in the same time, have the obvious advantage in cylinder head acoustic signal processing. After the cylinder head acoustic signal was denoised, the misfire and valve clearance related fault could be diagnosed through the cylinder head acoustic signal.