The invention discloses a method for designing an overall-width flat-inner-bottom liquid cargo ship, belongs to the technical field of shipbuilding, and relates to a structural design method of an inner bottom in a double-bottom ship and a bottom side tank area of a 
bilge area. The method comprises the steps that a flat outer bottom, the 
bilge area, a side, a bottom side tank cantboard, an inner shell and a flat inner bottom are parallelly arranged at the bottom of the ship, the flat outer bottom is connected to the side through the 
bilge area, the flat inner bottom is connected to the bilge area or the side through an extension flat inner bottom, and the flat inner bottom is connected to the inner shell through the bottom side tank cantboard. According to the method for designing the overall-width flat-inner-bottom liquid cargo ship, the flat inner bottom is connected to the side through the extension flat inner bottom arranged in the bottom side tank, so that the flat inner bottom and an extended bottom area form a 
void space, the weight of 
ballast water for the ship reaches or is close to a minimum value of a dedicated 
ballast tank required by the MARPOL convention, so that 
energy consumption is reduced, and the purpose of 
energy conservation is achieved. Because the flat inner bottom serving as the bottom of the 
ballast tank is more beneficial for removing 
sediment, and theusing convenience is improved; and the structural continuity of the flat inner bottom can better transmit load, 
stress concentration is reduced, and the structural strength is improved.