A quick-mounting device (1) for canisters comprises a housing (11) and an adjustment means (12). The housing defines a recess (112) at a top thereof and a hollow space therein (111). The housing is sealed with a cover (117) that defines a though hole at a center thereof. The adjustment means includes two arched pieces (124) and a control piece (121). The control piece has a pusher (122) and is placed on top of the arched pieces to engage with two ends of each arched piece. Thereby, the pusher of the control piece can be pressed to have the two arched pieces moved outwardly to change the dimension of the opening defined by the two arched pieces so as to mount a canister.