According to the relationship between vibration velocity and in-cylinder pressure for cylinder head of diesel engine, the extraction method of vibration velocity characteristic parameter for describing the in-cylinder peak pressure of diesel engine was deduced with calculus theory based on numerical and experimental research. The results show that the peak displacement extracted from vibration velocity is approximately linear to the peak pressure and the peak displacement can hence describe the change of in-cylinder peak pressure accurately. The experimental results indicate that the peak displacement calculated by vibration velocity has the same trend with in-cylinder pressure, which shows the approximate linear relationship. However, the linearity is slightly worse due to the influence such as interference information and fluctuations in operating conditions. The fitting linear relationship between peak displacement and peak pressure for different engines is conducted by using the data extracted under different conditions based on least square method, which can realize the quantitative evaluation of in-cylinder peak pressure based on the vibration velocity.