Repeat sun-synchronous orbit is one of the most important orbits for earth observation missions,because the orbit can satisfy the requirements of illumination and temporal resolution.Q value is the connection between sun-synchronous orbit and repeat orbit,so Q value plays an important role in repeat sun-synchronous orbit design.In this paper,the characteristics of sun-synchronous orbit and repeat orbit are analyzed,and the relationship of Q value and repeat sun-synchronous orbit is established.With the analysis of repeat orbit characteristics,a Q value design method of repeat sun-synchronous orbit is derived,and an optimal orbit is selected using the method.The selected orbit shows that the orbital altitude is 502.59 km,repeat period is 21 days and mean revisit period is 5 days,which is an optimal repeat sun-synchronous orbit for the revisit characteristic within the range from 500 km to 510 km.It concludes that this method finds the key points of repeat sun-synchronous orbit design in theory,and provides a credible and easy method for complex orbit design tasks.