A tissue paper cutting mechanism having upper knife with variable spiral curve angle includes a base, a bed knife roller rotatably mounted on the base and provided on a circumferential surface with a plurality of straight bed knives radially spaced at predetermined intervals, a knife carrier mounted on the base near the bed knife roller at an axis skew angle relative to the bed knife roller, and an upper knife associated with a structural surface of the knife carrier to extend along a longitudinal axis direction of the knife carrier from an end to another opposite end thereof in a spiral direction, such that an angle contained between an axis of the upper knife and a tangential line to the circumferential surface of the bed knife roller non-linearly varies with every change of the cutting position on the upper knife.