After analyzing the motion mechanism of human ankle, a new type of two DOF parallel ankle joint mechanism for a biped robot is designed in this paper. The parameterized model of the mechanism was established, of which the kinematic and dynamic analysis are also given. According to the dynamic characteristics of the ankle joint during walking, we optimize the structure parameters of the parallel mechanism through iterative calculation, which makes the driving peak power and speed of the two driving elements of the ankle joint tend to be equal to each other. Through the analysis of the experimental results for the biped robot's ankle joint motion, it shows that the peak-power used to drive parallel ankle joints is only about half of that used to drive serial joints. Based on the premise that the dynamic characteristic of biped robot ankle joints during walking are satisfied, it is beneficial to the reduction of bulk, weight and the energy consumption of the joints by using lower-power drivers.