Caesium atomic fountain clock is a primary frequency standard,which realizes the duration of second.Its performance is mostly dominated by the frequency accuracy,and the C-field induced second-order Zeeman frequency shift is the major effect,which limits the accuracy improvement.By applying a high-precision current supply and high-performance magnetic shieldings,the C-field stability has been improved significantly.In order to achieve a uniform C-field,this paper proposes a doubly wound C-field solenoid,which compensates the radial magnetic field along the atomic flight region generated by the lead-out single wire and improves the accuracy evaluation of second-order Zeeman frequency shift.Based on the stable and uniform C-field,we launch the selected atoms to different heights and record the magnetically sensitive Ramsey transition|F =3,mF =-1 > → |F =4,mF =-1 > central frequency,obtaining this frequency shift as 131.03 × 1015 and constructing the C-field profile (σ =0.15 nT).Meanwhile,during normal operation,we lock NTSC-F2 to the central frequency of the magnetically sensitive Ramsey transition |F =3,mF =1> → |F =4,mF =1> fringe for ten consecutive days and record this frequency fluctuation in time domain.The first evaluation of second-order Zeeman frequency shift uncertainty is 0.10 × 1015.The total deviation of the frequency fluctuation on the clock transition induced by the C-field instability is less than 2.6 × 10-17.Compared with NTSC-F1,NTSC-F2,there appears a significant improvement.