An auto-filling assembly for a refillable sprayer has a base device, a controlling device and a filling device. The base device has a base, an oil tank and a connecting jacket. The connecting jacket is mounted on the base and has a connecting disk, a switch and an engaging mount. The controlling device is connected to the base device and has an inlet valve, a first linking valve, a gas-controlling valve, a second linking valve and a triple valve. The filling device is mounted on the base device, is connected to the controlling device and has a storage bottle, a release valve and a mechanical valve. The auto-filling assembly can be used to fill pressurized gas and oil into a refillable sprayer conveniently and automatically.