To detect the shape of a Fiber Bragg Grating(FBG) sensor in a colon, a coordinate fitting method based on Frenet frame was proposed to improve the precision of the FBG curve reconstruction algorithm. Firstly, four FBGs perpendicular to each other were clinged to one Shape Memory Alloys(SMA) substrate. Hence, one FBG curve sensor with a diameter of 3 mm and a length of 900 mm was formed. Then, this sensor was put on a millimeter-square graph paper and a cylinder respectively to accomplish the reconstruction of 2D and 3D curves. Furthermore, every reconstructed value of data collection points was read from the curve reconstruction software and reconstruction errors were acquired by the contrast between the reconstructed values and the standard values read from the coordinate paper. Finally, the merit degree between two fitting methods was obtained. Experimental results show that the proposed coordinate fitting method based on Frenet frame effectively improves the reconstruction accuracy. When the FBG achieves their ultimate strain, the proposed fitting method can reduce the average error by 3.5%—5.5% at single direction, which lays a foundation for the precision upgrading of the FBG curve sensing system.