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The case that aggregation brings a huge influence on the structure and fluorescent property of 5-(4-(1,2,2-triphenylvinyl)phenyl)thiophene-2-carbaldehyde (P4TA) was detailedly investigated here by employing both quantum mechanics and molecular mechanics. Besides the isolated molecule, the aggregated molecule both in water and crystal state was studied by focusing on the comparison of photoelectronic properties, including the geometrical and electronic structures at ground and excited states, emission and internal conversation properties. For the aggregation state, the intermolecular interaction was employed to explain the difference in structure, emission color and intensity of different polymorphs. The noticeable contribution from low-frequency region, corresponding to the four phenyl rings twisting vibration, to the Huang-Rhys factor and reorganization energy, as well as the possible potential energy surface crossing between S0 and S1 states for isolated molecules was considered as the reason of its aggregation induced emission (AIE) performance. Importantly, the aggregation process in water simulated at the same time helps us to have a deeper understanding of the AIE behavior of P4TA, which also provides another perspective to explore the AIE phenomenon in theory.