In order to solve the on-line detection problem of insulator insulation resistance for UHVDC transmission lines, a new method of measuring insulation resistance of UHVDC line insulators by using live robots is proposed, and a low zero value insulator detection robot for transmission lines is developed. By using the method of equipotential operation, a bypass circuit for the tested insulator is built with the arms and body of the robot. The interference of leakage current to the tested circuit is eliminated and the measurement accuracy is improved by using this method. The measurement, storage and transmission of insulator resistance information are completed by using the mechanical structure design of the robot itself and the cooperation of hardware circuit. Meanwhile, the discrete fitting function is established to accurately calculate the data, thus the accuracy of the test results is ensured. Finally, a prototype is developed for comparative test. The test results show that the robot measurement method can accurately measure the insulators resistance in operation, and the error is controlled within 5%. Then, the operating condition of insulators can be judged, and the online warning of low-zero insulators can be realized.