The invention discloses a flow distributing method adopting melt extrusion pressure to drive full-engagement gears and a flat die head. The method includes the steps that a polymer melt from an extruder is distributed in the axial direction of the gears to fully occupy tooth grooves after meeting a barrier in an input area of a full-engagement gear set so as to force all the gears in the full-engagement gear set to rotate; the polymer melt is conveyed to an output area of the full-engagement gear set along with rotation of the gears; and therefore the extrusion speed is uniformly distributed in the axial direction of the gears. The flat die head comprises a die head transition sleeve, a die head seat and a bush die which are connected in sequence from top to bottom, wherein the full-engagement gear set is disposed in the die head seat, and a sealed space is formed between the outer contour of the full-engagement gear set and the die head seat. The polymer melt with uniform flow speed in the extrusion direction and the direction perpendicular to the extrusion direction can be provided at the same time, the pressure drop in flow channels is small through the method, the melt uniformity in the extrusion flow channels can be improved without an extra regulating rod, and high-quality uniform sheets or thin film materials can be produced directly through the melt.