A side buckle of swimming goggles is approximately formed into a strip shape. The side buckle has a retaining clip and a retaining groove respectively at two ends thereof. The retaining clip has two shaft rods and a step-shaped urging portion. The retaining groove has two shaft holes and a belt hole. A connecting portion is formed between the retaining clip and the retaining groove. The retaining clip and the retaining groove may be locked to each other after the connecting portion is bent. Thereby, the side buckle of swimming goggles is fitted at side edges of a pair of swimming goggles and connected to a belt. The connecting portion is pressed to trigger the operation of the retaining clip to loosen or fasten the belt, so as to quickly adjust a length of the belt.