An adjustable attachment device includes a clamping member for securely clamping a tubular member, a sleeve securely mounted to the clamping member, a ball received in the sleeve, a connecting seat having a portion securely coupled to the ball to move therewith, and a resilient member. The sleeve includes a longitudinal through-hole. Further, the sleeve includes a cutout passing through an upper end of the longitudinal through-hole. The resilient member biases the ball to press against a spherical upper wall section of the sleeve, thereby retaining the connecting seat in place. The connecting seat is pressable to compress the resilient member and cause the ball to disengage from the spherical upper wall section of the sleeve, allowing the connecting seat to move along the cutout of the sleeve and allowing the connecting seat to turn to a desired orientation relative to the tubular member.