The invention discloses a production method for PTT (Poly Trimethylene terephthalate)/PET (Poly Ethylene Terephthalate) side-by-side composite elastic short fibers. The production method comprises the following steps of (1) compositing a raw material a, namely, modified PET and a material b, namely, PTT in a large-capacity two-channel composite spinning assembly after respectively passing through crystallizing and drying equipment, melting and extruding equipment and a spinning manifold, obtaining a strand with a side-by-side shape, and then obtaining a PTT/PET side-by-side composite elastic short fiber precursor c after spinning, circular blowing cooling, oiling, rolling and falling into a barrel; (2) feeding the precursor c into a packaging machine for packaging after bundling, drafting, oiling, tension heat setting, oiling, mechanical rolling, relaxation heat setting and cutting. According to the production method disclosed by the invention, since the special composite spinning manifold, the large-capacity two-channel composite spinning assembly and a multi-roller tension heat setting instrument device are adopted, the PTT/PET side-by-side composite elastic short fiber prepared by the production method has moderate elasticity, excellent hand feeling and good stretch elastic recovery.