Octadecyl acrylate was proven to exist in rotator phases, and the mechanism of its chain-reaction photopolymerization was revealed. The polymorphic behavior of octadecyl acrylate was studied by differential scanning calorimeter (DSC) and X-ray diffraction, which concluded that octadecyl acrylate exhibits two rotator phases (R-II, and R-I), one orthorhombic crystal phase (C-ort), and one triclinic crystal phase (C-tri) phase. The chain-reaction photopolymerization of four phases of octadecyl acrylate were studied by photo-DSC, and the theoretical possibilities of one-dimension chain propagation in R-II R-I, and C-ort phases were analyzed by using the molecular dynamics simulation results. Combining the experimental and calculation results, the chain-reaction polymerization mechanism either intralayer or interlayer was discussed and disclosed. The question of whether the chain-reaction polymerization of octadecyl acrylate can occur in crystal was answered, and the reason was explained.