A quick-detachable connecting device with a simplified thread structure includes a top nozzle, a connecting tube sleeve, a locking tube and a locking piece. The top nozzle is provided with a through hole, in which the connecting tube sleeve is assembled. The locking tube is accommodated in the through hole. An outer peripheral surface of the locking tube is provided with a second stopping structure, and an inner peripheral surface of the connecting tube sleeve is provided with a first stopping structure fitting the second stopping structure. The locking piece is movably arranged on the locking tube for circumferentially limiting or releasing the locking tube. A connection method using such connecting device is also provided.