Engineering practice indicates that the use of bentonite support fluid in borehole may significantly reduce the pile bearing capacity. To estimate the reduction quantitively, the static load test in field was performed on two kinds of piles with and without bentonite fluid, i.e., the slurry displacement (SD) pile and continuous flight auger (CFA) pile. Test site was selected in Huangpu District of Guangdong, and two CFA piles and one SD pile were installed. Sensing technology of ultra-weak fiber Bragg grating was applied to measure the soil-pile interface behavior, with a series of sensors attached to the longitudinal steel bar. Evolution of shaft resistance with the static load was thereby captured. Test results show that the bentonite support fluid reduces the shaft resistance by 52 and 61% in granitic residual soil and completely decomposed granite soil, respectively. Field excavation suggests that the reduction is attributed to the existence of residual bentonite layer between the pile concrete and soil, instead of the filter cake formed by bentonite filtration.