In astronomical detection, the measurement precision of radial velocity in Earth-like planet detection is required to reach 10 cm/s. The spectrum drift caused by circular fiber scrambling is becoming a major factor that influences the measurement precision of radial velocity. The sectional fiber transmission systems of circular fiber combined with polygonal fiber are proposed to improve the scrambling property of circular fiber. The spot quality of fiber output field is evaluated by an optical system for testing spot quality of far field and near field. The far field and near field distributions of sectional fiber transmission systems, which are consisted of circular fiber, circular fiber combined with square fiber, and circular fiber combined with octagonal fiber, are studied under different coupling conditions. The scrambling gains and energy variations of these fiber transmission systems are analyzed. The results show that the scrambling property and the system stability are improved by the sectional transmission system of circular fiber combined with polygonal fiber. The coupling energy loss is caused by large incidence shift on the condition that the core diameter of polygonal fiber is smaller than the core diameter of circular fiber, and the conservation of the energy holds on when the core diameter of polygonal fiber is larger than or equal to the core diameter of circular fiber. The transmission system of circular fiber combined with octagonal fiber, which is with a small mass center offset in the near field and a large scrambling coefficient, can decrease the spectrum drift induced by coupling error, and increase the measurement precision of radial velocity.