The synergistic effect of polyethylene glycol (PEG) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) can effectively reduce the protein absorption, which is beneficial to theranostics. However, PEG–PMPC-based polymers have rarely been used as nanocarriers in the theranostic field due to their limited modifiability and weak interaction with other materials. Herein, a plain method was proposed to endow them with the probable ability of loading small active agents, and the relationship between the structure and the ability of loading hydrophobic agents was explored, thus expanding their applications. Firstly, mPEG–PMPC or 4-arm-PEG–PMPC polymer was synthesized by atom transfer radical polymerization (ATRP) using mPEG-Br or 4-arm-PEG-Br as the macroinitiator. Then a strong hydrophobic segment, poly(butyl methacrylate) (PBMA), was introduced and the ability to load small hydrophobic agents was further explored. The results showed that linear mPEG–PMPC–PBMA could form micelles 50–80 nm in size and load the hydrophobic agent such as Nile red efficiently. In contrast, star-like 4-arm-PEG–PMPC–PBMA, a monomolecular micelle (10–20 nm), could hardly load any hydrophobic agent. This work highlights effective strategies for engineering PEG–PMPC-based polymers and may facilitate the further application in numerous fields.