A decorative wall tile mold for the in-situ production on a building, wherein the mold, which can be demolded twice, is a sheet with pierced work, comprises the following detachable lateral structures: an insider layer: a bottom film is set at the bottom surface thereof, the bottom film and the building have a first combinative force; an outer layer: a surface film is set on the exterior surface thereof, a self-adhesive glue section is set up between the inside layer and the outer layer, which makes the outer layer and the inside layer have a second combinative force, and the second combinative force is less than the first combinative force. A method for the in-situ production of decorative patterns on a building is also provided.