As an injectable and biodegradable hydrogel, thermogels of amphiphilic block copolymers of polyester and polyether in water show great potential in biomedical fields. It is challenging to reveal the mechanism behind the reversed thermogelling with sol-gel transition upon heating. Herein, a computer simulation and corresponding experiments are combined to examine aqueous systems of amphiphilic diblock copolymer of methoxypoly(ethylene glycol) and poly(D,L-lactide-co-glycolide). We synthesized the copolymer via ring-opening polymerization and characterized the thermogelling behavior of its aqueous solution by 3D dynamic light scattering and diffusing wave spectroscopy. Fluorescence resonance energy transfer and C-13 NMR spectroscopy etc. were also adopted to explore the structure change during thermogelation. A dynamic Monte Carlo simulation was performed for a corresponding multichain system. A new type of micelle, the semi-bald micelle, was first proposed as the precursor for thermogelling and was confirmed from both simulations and experiments. We demonstrate that the thermogel structure is a percolated micelle network with hydrophobic channels that evolved from the semi-bald micelles. The thermogelling mechanism is discussed at the chain level. On the basis of the mechanism study, we put forward the molecular design principle of the thermogels.