The distance between tip and sample(T-S) can be regulated using piezo-electric quartz fork glued with micro optic fiber probe.The principle of this technology is studied.The biquadrate vibration equation for fork-probe-sample system is established,the relations between the electric current of the fork and T-S distance (I-d curves) are analysed theoretically,and the mechanism model of the shear force between the optic fiber probe and the sample is established.In this work the I-d curves of various samples under various conditions are measured.The results accord with the theoretical analyses very well.Furthermore,the intrinsic reasons for shear force damping between T-S have been revealed i.e.the water and hydrocarbon coupled between optic fiber probe and sample due to the capillary cohesion force.