The soft base in the testing section of Jiang-zhu highwaysis 55 meters deep, and its mechanical property is not better than that of silt soil, the sand-wick drained length is one of the crucial factors for this highwaywhich influences the consolidation speed. Based on the characteristic of soft foundation, experiences and the interrelationship between the processing area and the subjacent bed effecting on draining, the mechanism of extra-long sand-wick for the extra-deep soft foundation is studied. The theoretical consolidation degree in the deep soft foundation treating with extra-long sand-wick drain had been calculated, the designed depth of sand-wick drain were 25 and 22 meters. Moreover, the total settlement will be predicted. According to the monitoring data of displacement and settlement during loading, the characteristic of consolidation settlement of the soft soil foundation had been analyzed. The results indicated that the settlement was very sensitive to the loading speed. The sedimentation became fast with the increasing loading. The sedimentation speed was always very high, which was 15 to 25mm/day on the average. During the prophase of pre-compaction, the consolidation degree of section K53+242.47 (h=25m) was up to 80% in 350 days, which was much faster than the section K53+402.47 (h=22m). Moreover, the soft soil foundation was consolidated stably according to the strict dynamic monitoring data. By using hyperbolic theory to analyze monitoring data, the predictive settlement value of post-project can meet the designed requirement. The treating effect of the 25m depth sand-wick drain was better than that of 22m, which provided available data for extra thick soft foundation treatment.