Abstract: Accurately determining the unit friction in pile driving is very important for improving the accuracy of pile driveability analysis. Based on the in-situ cone penetration test data and high strain dynamic test results of an interlayered soil site, the change of unit friction during pile driving is studied. The results show that during the pile driving, although the total friction of pile keeps increasing with increment of penetration depth, and the total friction at the same penetration depth decreases, which is the same as that in ordinary site, the interlayered soil has a significant effect on the change of unit friction with depth. The results of analysis shows that the unit friction also decreases significantly in sand layer and silt layer in the interlayered site, which could not be described correctly with the existing semi theoretical method. In view of this, a calculation method of reduction factor in clay based on sensitivity is proposed. At the same time, it is pointed out that the same reduction factor as that of clay should be used for sandy layer and silt layer for the interlayered soil. The unit friction obtained by the proposed method is basically consistent with the measured value. The proposed method could be used in practice to improve accuracy of pile driveability analysis in an interbedded soil site.