The steady state with duty cycle time 0.3-0.5 of Chinese fusion engineering testing reactor (CFETR) is a big challenge. To realize this goal, the divertor should be reliable. At the preconceptual phase of CFETR design, reliability engineering is engaged to determine how reliable of divertor is and how much level of reliability of divertor should be. The plasma facing components (PFCs), base plate, and support frame have been considered during the reliability analysis for the divertor modules. The relation of the reliability between a divertor system and its modules has been analyzed. The relations between steady-state operation with duty cycle time 0.3-0.5 and the reliability of divertor is studied in this paper. How the reliability of components, modules, and divertor system affects the duty cycle time of CFETR is also analyzed. To meet the requirement of duty cycle time, the lowest reliability level of divertor system is 0.3679 and its mean time to fail is only 1 year. The divertor system with such low reliability requires that each module should have a relative high reliability of 0.9794, and then, the PFCs should have higher reliability of 0.9930. These results are then must become the requirements for the engineering design of CFETR divertor in the future.