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This paper reports a new microfluidic device which can actively fine-tune the size of emulsion droplets in liquids by utilizing a controllable moving-wall structure. Furthermore, it can generate tunable double emulsion microdroplets with various sizes of external and internal droplets. Three pneumatic side-chambers were placed at T-junction and flow-focusing channels to construct the controllable moving-wall structure. Compressed air was used to activate the moving-all structure. The deformation of the controllable moving-wall structure can physically change width of the microchannel. Therefore the flow velocity can be locally changed by applying compressed air pressure. Size of the internal droplets at the intersection of the T-junction channel was then fine-tuned. Then the internal droplets were focused into a narrow stream hydrodynamically and finally chopped into double emulsion droplets using a pair of pneumatic moving-wall structures downstream. For single emulsion, the average droplets size of the oil-in-water emulsion could be actively fine-tuned from 50.07 to 21.80 mum under applied air pressures ranging from 10 to 25 psi with a variation less than 3.53%. For double emulsions, size of the external and internal droplets sizes can be fine-tuned successfully.