A device has been invented for protein crystallization by sandwiching the liquid droplet between two surfaces, in which both hydrophilic and hydrophobic surfaces can be used as crystallization substrates. Comparing with the traditional hanging drop method, it can also reduce the evaporation rate of the liquid droplet and provide a stable environment for the crystal growth. In this work, crystal growth experiments for several proteins, especially on the hydrophilic substrate of mica, have shown the positive effect on crystal growth for improving crystallization conditions and the quality of crystals. The features of this new sandwich method and its mechanism have also been discussed.