Soft robot arm driven by pneumatic artificial muscles can possess the ability of high ratio of power to weight, important for performance and light weight, and a vital component of the inherent safety approach to physical human-robot interaction. One of the main drawbacks of pneumatically actuated soft arm is that their stiffness cannot be varied independently from their position in space. Based on these reasons, a novel variable stiffness soft robotic arm composed of both three contractile and one extensor pneumatic artificial muscles was presented. This arm combined the light weight, high ratio of power to weight and robustness of pneumatic actuation with the adaptability and versatility, and stiffness can be adjusted independently of its length. Experiment platform of single contractile and extensor pneumatic artificial muscles was setup, and the static characteristic was identified for contractile and extensor pneumatic artificial muscles through quasi-static experiments. By using the least square method, the relational model of pressure, distance and stiffness for single contractile and extensor pneumatic artificial muscles was established. In order to analyze the stiffness characteristic of this arm, stiffness model of the designed soft arm was established. Stiffness experiment platform of this soft arm was setup. Experiment data was compared with theoretical model, and they possessed the same trend, the mean relative error was 3.60%, and the maximum relative error was 6.17%.