Almost all of polymeric nanostructures are now prepared by a classical post-self-assembly strategy. An ambitious target is to quest new polymer in situ self-assembly ways. Here we pose a light-initiated in situ self-assembly method, named LISA, which can de novo form various nanostructures through orthogonal photoligation from multiple homopolymers. Tuning the initial componential or light parameter, one can predict and govern the systematic morphological evolution. This method circumvents tedious synthesis and solution-processing procedures of block copolymers in post-self-assembly strategy and allows the preparation of ordered assemblies from simple homopolymers in one-pot photoreaction. We anticipate that this photocontrolled in situ self-assembly strategy would provide a new vision for facile construction of high-ordered nanostructures starting from homopolymers