The invention discloses a double-in and double-out coal mill thermal power generating unit instantaneous fuel optimization system and method. The system comprises a fuel control unit, an instantaneous fuel optimization calculation and conversion unit, a normal operation working condition coal quantity closed loop measuring unit and an abrasive belt load stopping memory storage unit, wherein the fuel control unit is used for receiving a control instruction sent by a boiler and an acceleration signal of a variable load, calculating a control signal of a coal mill capacity air door, and sending a result to the instantaneous fuel optimization calculation and conversion unit; the instantaneous fuel optimization calculation and conversion unit is used for judging a conversion condition through operation states of a corresponding coal mill and coal feeder, and transmitting a result to the fuel control unit; the normal operation working condition coal quantity closed loop measuring unit is used for forming an instantaneous fuel quantity correction factor through a theoretical coal quantity which is linearly determined by the capacity air door and a coal feeder instantaneous coal quantity and according to a difference between the quantities, and sending a result to the instantaneous fuel optimization calculation and conversion unit; and the abrasive belt load stopping memory storage unit is used for receiving a result of the normal operation working condition coal quantity closed loop measuring unit.