An auxiliary mounting device for linear rails includes a connecting rail and a ball-filling rail. Both sides of the connecting rail each have a first auxiliary groove. The ball-filling rail is formed on one end of the connecting rail and has two swinging arms and a flap. The two swinging arms are extending from one end of the connecting rail. The two swinging arms each have a second auxiliary groove and a pivoting portion. The two second auxiliary grooves correspond to the two first auxiliary grooves respectively. The two pivoting portions are formed between the two swinging arms and the connecting rail. The flap is provided between the two swinging arms. The flap has two lateral edges and an inward-folding crease. The two lateral edges connect both sides of the flap to the two swinging arms. The inward-folding crease is formed in the middle of the flap.