The invention relates to a supercritical one-time diverting type longitudinal three-stage Rankine cycle power generation system. The system comprises an LNG (liquefied natural gas) circulating pump and a sea water pump and also comprises a primary power generation unit, a secondary power generation unit and a tertiary power generation unit which are sequentially distributed in longitudinal arrays,wherein the primary power generation unit comprises a primary evaporator, a primary steam turbine, a primary condenser and a primary working medium pump which are connected sequentially to form a closed loop; the secondary power generation unit comprises a secondary evaporator, a secondary steam turbine, a secondary diverter, a secondary condenser, a secondary mixer and a secondary working mediumpump which are connected sequentially to form a closed loop; the tertiary power generation unit comprises a tertiary evaporator, a tertiary steam turbine, a tertiary diverter, a tertiary condenser, atertiary mixer and a tertiary working medium pump which are connected sequentially to form a closed loop. The supercritical one-time diverting type longitudinal three-stage Rankine cycle power generation system has the advantages as follows: the system is simple in structure, convenient to use and capable of being widely used in LNG cold energy power generation field.