Choice the robot for making the automation H-beam cutting system among some candidate robots. Although the H-beam is in the work envelop of chose the robot, sometimes the work is impossible. So, find the special work envelop when the robot is working for special requirement position like H-beam cutting. The special work envelop can find using inverse kinematics. But it needs the very high performance and complicated algorithm with computer. So, I decide that combine the way of inverse kinematics and forward kinematics. I have reduce the calculation time and process compare with just use inverse kinematics. To do, the first step is making the forward kinematics program. The second step is finding the dominant position for the H-beam cutting. And then make the robot maintain that position. The coordinates of wrist and end-effector of the robot can be calculated using forward kinematics program. And then separate the robot into two parts. the fiducial point is wrist of robot. Finally move the robot of upper part with same distance. And on each coordinates, analytic inverse kinematics of lower part. To verify the work envelop which is found through the way of this paper, use the program IGRIP of robot simulation tool. The result has verified fall out good.