Abstract: The enhanced and weakened effect of skin friction near the bottom of piles is analyzed and the characters of skin friction varying with intensity and stiffness of bearing stratum are obtained based on the results of the static load and stress tests on cast-in-situ piles by use of different construction technologies at the same site.The mechanism of the enhanced and weakened effect and that of transforming from the weakened effect to the enhaned one are discussed qualitatively based on the results of the static load tests on abnormal piles under large settlement.The skin friction is further known to be a non-fixed value,which is beneficial to the reasonable design and construction of cast-in-situ piles and reinforcement of abnormal piles.