According to geometrical relationship between discontinuities and measuring window and referring to H-H method for trace length estimation，a new expression is proposed to express probability relationship between trace length of discontinuity and its orientation. Base on Laslett method for trace length estimation，the generalized H-H method for trace length estimation is put forwad，which overcomes some disadvantages of H-H method and extends its area of applications. Using this method，an application example of rock engineering is analyzed.