The present invention provides a square all-purpose building block used for an outdoor building block. The square all-purpose building block is composed of an upper cover and a lower cover, wherein structures of the upper cover and the lower cover are the same, either of the upper cover and the lower cover comprises a cover plate, a side plate and a round tube, the cover plate is of a square structure, outer surfaces of the cover plate and the side plate are provided with non-skidding coatings, the non-skidding coatings are provided with grains for preventing the cover plate from being scratched, a round hole is disposed in the center of the cover plate, the diameter of the round tube is the same as that of the round hole, one end of the round tube is fixedly installed on the round hole, the side plate and the cover plate are perpendicular, the side plate is arranged around the edge of the cover plate, the side plate and the cover plate are fixedly connected into an integral body, both sides of the side plate are provided with clamping rectangular slots sinking towards the round tube direction, and connection parts of the a bottom face and side faces of the clamping slots are provided with circular arc chamferings and slope transitions. Defects of the prior art are solved, and the building block is simple and secure in structure, light in weight and not easy to deform.