In order to solve the problem that the clock?offset measurement over a long fiber link is limited to picosecond level, we have completed the experimental study on clock?offset measurement over a 100 km fiber link based on dual?comb linear optical sampling method. The net dispersion of long?distance fiber link and the line width of comb teeth are optimized by using dispersion compensation and comb compression technology. The central time of interferograms can be accurately obtained, and the clock?offset is measured with higher resolution. The time delays over fiber link measured with time interval counter and dual?comb linear optical sampling method are compared, and the clock?offset over a 100 km fiber link is measured. The experimental results demonstrate that the clock?offset measured based on the linear optical sampling method is less than 100 fs. High resolution clock?offset measurement is the key technology of long distances time synchronization. High precision time synchronization technology has important applications in accurate navigation, high speed communication and timing system.