This paper discusses a design of a flexible bending biomimetic octopus arm unit. Firstly, bending function of the octopus arm is analyzed. Secondly, the structure of the arm unit driven by shape memory alloy (SMA) is presented which can realize flexible bending. Meanwhile, mechanical model of the arm unit is established to find the proper parameters of the arm unit. Finally, experiments are carried out to demonstrate the feasibility of the structure.