A method for estimating posture of robotic walking aid(1) includes: providing a motor controller(50), a motor encoder(71) and a motor(70) on right and left hip joints, and right and left knee joints of the robotic walking aid(1), providing an inertial sensor(60) on upper body(40) of the robotic walking aid(1), wherein the motor controller(50), the motor encoder(71), the motor(70) and the inertial sensor(60) are coupled to a control unit(80); installing the robotic walking aid(1) on a user; inputting the lengths of the upper body(40), two thighs, two shanks, two feet of the robotic walking aid(1) to the control unit(80), wherein the upper body(40), two thighs, two shanks, two feet form a plurality of points; obtaining an angle of the upper body(40) corresponding to a reference frame with the inertial sensor(60); obtaining angles of those joints with those motor encoders(71); and calculating 3 dimensional coordinates of each point with a motion model.