Assembling method of flattening-realizable file rack, comprising following steps: gluing; grey board pasting; folding; opening-burying; and forming. File rack assembled from the flattening-realizable file rack assembly method, comprises first single-powder cardboard body, wherein second single-powder cardboard body is formed at left side of the first single-powder cardboard body, third single-powder cardboard body is formed at right side of the first single-powder cardboard body, fifth single-powder cardboard body is vertically formed at top end of the first single-powder cardboard body, fourth single-powder cardboard body is formed at left side of the fifth single-powder cardboard body, and bottom end of the fourth single-powder cardboard body is connected with top end of the second single-powder cardboard body, sixth single-powder cardboard body is formed at right side of the fifth single-powder cardboard body, and bottom of the sixth single-powder cardboard body is connected with top end of the third single-powder cardboard body for forming.