A collar structure for animal leashes includes a lead having a hoop disposed at one end thereof for a ring to be attached thereto, and a clamping piece made up of a clamping plate protruding upwards at the top side of a fixing body and supported by a spring element wherein the clamping plate of the clamping piece is pushed downwards to align a pair of symmetrical left and right clamping holes disposed thereon with a pair of matching left and right fixing holes of the fixing body thereof, and a free end disposed at the other side of the lead thereof is sequentially guided through the left fixing hole and the left clamping hole thereof before extending further to wind around the other side of the ring thereof and bend backwards to pass through the right fixing hole and the right clamping hole thereof respectively so as to form a U-shaped adjustable section and a movable collar to fit an animal's neck. Therefore, via the clamping piece to clamp tight the lead for secure location thereof, the movable collar can be stably held in place in a properly adjusted size without getting tightened or loosened by the animal squirming or nuzzling therein so as to prevent the animal from getting choked or injured and avoid the risk of escape thereof, efficiently providing a more convenient and safer collar structure for use.