The invention relates to a modified polyacrylonitrile-based carbon fiber protofilament of attapulgite nano particles and a preparation method thereof. Polyacrylonitrile copolymer and the attapulgite particles are mixed and dissolved in organic solution to prepare 20 to 25 percent by weight of spinning dope for preparing the modified polyacrylonitrile-based carbon fiber protofilament containing 0.1 to 2 percent by weight of attapulgite. The protofilament has 3 to 4.5dtex of fibrousness, 6 to 9 cN / dtex of stretching strength and 10 to 12 percent of elongation at break. And the attapulgite is evenly distributed in the polyacrylonitrile protofilament. Compared with the pure polyacrylonitrile protofilament, the strength and the elongation at break of the attapulgite modified polyacrylonitrile protofilament are obviously enhanced.