A three-dimensional printing system includes a build device and an optical projection engine. The build device includes a curing tank, a photocurable material and a build platform, and the photocurable material and the build platform is disposed in the curing tank. The optical projection engine has a zoom lens for projecting image beams with at least a first pixel size and a second pixel size on the build platform to cure the photocurable material, and the first pixel size is different to the second pixel size.