Knock feature extraction detection is the key of closed loop control of ignition in gasoline engine, and it is also the difficult point in knock measurement. In this paper, an investigation of knock detection in gasoline direct injection(GDI) engines using bispectrum slice based on the engine cylinder head vibration signals. Power spectrum density(PSD) estimation is used to determine the band range of the resonance frequency generated by knock component. The diagonal slice of bispectrum is computed to extract the non-linear feature deriving from the quadratic phase coupling, as well as the knock characteristic frequencies. This method is endowed with characteristics of suppressing Gaussian white noise, detecting the nonlinear coupling feature, and which is used to extract the knock threshold characteristic information. Derived from the above method, a novel knock intensity factor has been developed and the relevance of the proposed criterion for characterizing different levels of knock has been investigated.