When the Three Gorges reservoir water level changes, the second chamber of five stage lock of TGP will be required to operate as the firstclass and middle class chambers in every year.Under the above two operational conditions,the filling volumes of chamber are almost equal,but their difference of initial water heads is twice as much,so the size of the conveyance system cannot at the same time meet the requirements of both filling time and mooring forces. In order to satisfy the design requirements,an optimized operationalmode was adopted through a model test research with a scale of 1∶30,which was to use the conveyance culvert to supplement water into the third chamber so as to keep the water level of the second chamber being not below than 133 m. The results show that the filling time is not over 12 min,and the mooring forces are less than allowable values.