In this paper, a micro bending robot (MBR) for an active catheter which allows any bending direction and bending angle and has a simple structure for miniaturization was developed. The MBR is 1.25 mm in outer diameter. In order to achieve a large bending angle meanwhile not to increase the outer diameter of the MBR too much, the MBR consists of 3 bending units. A bending unit consists of a bias spring and 3 incorporating shape memory alloy (SMA) coil actuators. The bending model of a bending unit was established based on a design chart and equation. Kinematics of a bending unit was analysed, and the position and orientation of the tip center and the displacements of all 3 SMA coil actuators of the bending unit in any bending direction and bending angle were obtained. Finally, the position and orientation of the tip center of the MBR and its bending angle in any bending direction were deduced. The works of this paper help to consummate the design theory of active catheter and has important theoretic significance and practical applied value.