The atmospheric infrared ultra-spectral sounder (AIUS) is one of the main loads of GF-5 satellite. It is used to obtain the vertical distribution information of various atmospheric trace gases through solar occultation observation. And accurate spectral calibration is the key and foundation for its data quantitative inversion. Ultra-spectral resolution and no matching on-board spectral calibration equipment make it is difficult to realize high precision on-orbit spectral calibration for AIUS. A multi-line linear fitting algorithm based on atmospheric characteristic lines is proposed, which employs the key technologies such as Doppler shift correction, lines selection and accurate peak position determination to improve the spectral calibration accuracy. And after the successful launch of GF-5 satellite, a series of spectral calibrations and accuracy analysis are carried out for AIUS. The results show that this algorithm can achieve high precision spectral calibration of AIUS. The mean absolute deviations of MCT and InSb channels are 0.00437cm^-1 and 0.00389cm^-1, which are less than 0.008cm^-1 required by trace gas inversion application.