As a kind of thermo-responsive hydrogel, amphiphilic block copolymers are widely investigated. However, the molecular mechanism of their structural change during the gelation process is still limited. Here, a well-controlled triblock copolymer poly(N,N-dimethylacrylamide)-b-poly(diacetone acrylamide)-b-poly(N,N-dimethylacrylamide) (PDMAA-b-PDAAM-b-PDMAA) was synthesized. Its optical microrheology results suggest a gelation temperature range from 42 to 50 °C, showing a transition from viscosity to elasticity. The morphological transition from spheres to worms occurs. Temperature-dependent IR spectra through two-dimensional correlation spectroscopy (2D-COS) and the Gaussian fitting technique were analyzed to obtain the transition information of the molecular structure within the triblock copolymer. The N-way principal component analysis (NPCA) on the temperature-dependent NIR spectra was performed to understand the molecular interaction between water and the copolymer. The intramolecular hydrogen bonds within the hydrophobic PDAAM block tend to dissociate with temperature, resulting in improved hydration and a relative volume increase of the PDAAM block. The dissociation of intermolecular hydrogen bonds within the PDAAM block was the driving force for the morphological transition. Moreover, the hydrophilic PDMAA block dehydrates with temperature, and three stages can be found. The dehydration rate of the second stage with temperature from 42 to 50 °C was obviously higher than those in the lower (first stage) and higher (third stage) temperature ranges.