A polythiophene derivative with conjugated side chain were synthesized and characterized by LB method. It was found that this polymer can form stable monolayer at the air/water interface. It is important that the interaction between the polymer chains showed important role in regulating the spectra and interfacial morphology of polythiophene polymer, which have been characterized by spectral and morphological investigation. The absorption spectra of the polymer show two absorption peaks located in UV and visible region, where the visible absorption peak is attributed to the -* transition of the conjugated main chains while the UV absorption peak comes from the conjugated side chains. The necklace-like domains could be found and altered with change of surface pressure. The present work provided new insight into the design and interfacial assembly of polythiophene derivative with special molecular structures and substituted groups.