A metal mask plate and a fabrication method thereof. The metal mask plate has a first main surface and a second main surface opposite to each other, the metal mask plate includes: a plurality of panel-defining regions spaced apart; and a main frame portion. In each of the panel-defining regions, a plurality of via holes configured for defining a pixel structure are arranged in a matrix, and each of the via holes communicates the first main surface and the second main surface of the metal mask plate, side walls of every two adjacent via holes are directly connected with each other on the second main surface of the metal mask plate so that the metal mask plate between the two adjacent via holes (101) in the panel-defining region has a thickness D1. The main frame portion of the metal mask plate has a second thickness D2, and the first thickness D1 and the second thickness D2 satisfy |D1−D2|/D1≤20%. Thus, a wrinkle and deformation caused by the difference of the thicknesses during unfolding can be effectively avoid, so as to further effectively improve quality of the OLED display product fabricated by adopting the fine metal mask plate.