In order to realize the function that mobile robot can detect environment and performance it intuitively, the simple geometric map rendering system in front of the robot around 180 degree has been designed. lpc1768 has been used as the core. The small servo MG90S and the ultrasonic module HC-SR04 have been seen as the base of the machine. By the program written, the PC can process data measured by the machine. And the two-dimensional map of the environment in front of the robot would been displayed through window interface written. Through three types of obstacles’ detection, the experimental result can been verified. It shows that the system can accurately reflect the environmental information in front of the robot, so real-time map building can be done for mobile robot.