Abstract: The ground of 27m thick mucky and silty clay treated with 30m deep prefabricated drain in combination with preloading was discussed in the paper. The test results of foundations of two oil tanks of 50 000m~3 which had been preloaded by 18m thick fill embankment and retested by 17m water-filling show that the effective depth of prefabricated drains can reach 30m,the bearing capacity can reach 340kPa after preloading,the maximum settlement of the ground is 3. 36m and the degree of consolidation reaches 95%.The tests also show that the pore water pressures resulted from filling are rather small in the thick soft clay with many silty sandy intercalations,so the safety monitoring of the ground should be carried out according to the rates of settlements and lateral displacements. Due to the rapid dissipation of pore pressure,the fill can be continuously proceeded to top in layers of 30cm thickness at a loading speed of 1. 5 to 2 kPa/d.