Abstract: An electron linac was built at Institute of Modern Physics, Chinese Academy of Sciences, dedicated for high energy electron radiography experimental study. In order to satisfy the requirement of the experiment and stable linac operation, a low level radio frequency control system was designed, which achieved phase feedback control by using up-down converter and IQ demodulation techniques. In this paper, the design principle and digital algorithm of the low level radio frequency control system were introduced and the result of the closed loop test was given. The low level radio frequency control system achieves phase stability control accuracy of ±0.5° (peak to peak value) and 0.110 8° (root mean square), meeting the design goal. The low level radio frequency control system was built with commercial analog microwave device and mature commercial PXI card, and the control program was realized with LabVIEW software. The system has the characteristics of fast building, easy developing and realizing, compact structure and convenient use and maintain.