The thick soft marine clay of Hong Kong-Zhuhai-Macau Bridge(HZMB) ground was reinforced by sand compaction pile (SCP) method. A large scale plate load test was designed for evaluate the bearing capacity and deformability of the SCP composite ground. Better settlement result was obtained by utilizing the high sensitivity liquid level sensor system underwater. The research results show that the pile-soil stress ratio of SCP composite ground is closely related to load level that decreased with the load level increasing. On the same level, the ratio gradually reduced to a certain value finally. Four computing theory including Brauns’ method, Wong formula, Hughes & Withers formula and passive earth pressure method were discussed in bearing capacity calculation of SCP composite ground. The results by passive earth pressure method agreed best with the field experimental data. For this high-replacement ratio SCP composite ground, it is demonstrated that the settlement calculated with a reduction factor which was derived from Technical code for ground treatment of building is most close to the measured result; while there is significant difference when the overseas Coastal Area Development Institute of Japan(OCDI)’s experiential formula is employed. This research results provide useful data for the marine SCP composite ground design and deep sea in-situ test.