Analyses have been done to explain the generation of the Sagnac effect in time transfer system over optical fiber. Mathematical expression of the time deviation, which is caused by Sagnac effect between any two points on the surface of the earth, is deducted in detail. Influences on the accuracy of the time transfer are studied deliberately by simulation. It is demonstrated that the time deviation is decided by the latitude and the course of the fiber link and it may be larger than 1 ns for a long haul. So accurate time delay compensation or calibration should be concerned in terrestrial time transfer system.