Abstract: This paper introduced the monitoring on seepage for TGP’s left power plant building (left non-spillway) monolith. The monitoring result demonstrated that the osmotic pressure, seepage uplift and seepage quantity in dam body increased visibly after upper and lower coffer wall removal and water storage, but under design requirement. The result provided important scientific reference for the safety in stage of construction and operation of TGP. Moreover, it will optimize the design of TGP.?