Tuning the amphiphilicity of block copolymers has been extensively exploited to manipulate the morphological transition of aggregates. The introduction of crystallizable moieties into the amphiphilic copolymers also offers increasing possibilities for regulating self-assembled structures. In this work, we demonstrate a detailed investigation of the self-assembly behavior of amphiphilic poly(ethylene glycol)-block-poly(l-lactic acid) (PEG-b-PLLA) diblock copolymers with the assistance of a common solvent in aqueous solution. With a given length of the PEG block, the molecular weight of the PLA block has great effect on the morphologies of self-assembled nanoaggregates as a result of varying molecular amphiphilicity and polymer crystallization. Common solvents including N,N-dimethylformamide, dioxane, and tetrahydrofuran involved in the early stage of self-assembly led to the change in chain configuration, which further influences the self-assembly of block copolymers. This study expanded the scope of PLA-based copolymers and proposed a possible mechanism of the sphere-to-lozenge and platelet-to-cylinder morphological transitions.