High density digital video disc (HD-DVD) is one of the high density optical data storage technologies to meet the demand of high definition TV. The knife-edge method for focus error detection in HD-DVD driver is proposed in this paper. The calculation model for optical intensity distribution on photo detector under different defocusing amount is deduced on the basis of Fresnel formula and scalar diffraction theory.The S curve obtained theoretically showing how the focus error intensity varies with defocusing amount is in agreement with the experimental one for HD-DVD driver,which validates the calculation model.Furthermore, the focusing noise caused by pit-modulation in HD-DVD driver is analyzed and indicated to be less than 4.5%. It is concluded that a satisfactory focus range of linearity is realized in HD-DVD driver with knife-edge method which has high sensitivity and low focusing noise.