Based on the covering dimension method for measuring the San Andreas fault system applied by P.G.Okubo,the fractal behavior of Haiyuan Quaternary active fault system has been investigated.The fractal dimensions for the entire fault system and its two of the four segments,Jingtai and Haiyuan,are obtained,i.e.D_QB=1.137,D_OJ=1.109 and D_OH=1.182.The fractal dimensions for the other two segments,Shaojiashui and Lijunpu,aren't obtained,so the SL segments are self-affine or nonlinear fractals rather than self-similar ones.It.may be seen that the real seismic faults are more complicated than ideal ones,should be multifractals or self-affine.Basides.the correlation between fault geometry and seismicity,mechanical circumstance around the fault system has been emphatically discussed.It further proves that the geometry complexity of self-similar faults is close to seismic activity;shear faults have lower fractal dimensions (1.1-1.3);but the fault systems formed in tensional circumstance have higher fractal dimensions (1.5-1.6).Finally,this paper discussed the effect of fault trace map,etc.on the measuring accuracy of fractal analysis.