In order to implement the frequency offset estimation in direct sequency spread spectrum (DSSS) system based on the standard of IEEE 802.15.4k under low SNR circumstance, an improved frequency offset estimation algorithm with high accuracy and stability is proposed in this paper. The proposed algorithm firstly makes biased auto-correlation and iteration to the signals by using the phase and amplitude information at the same time under the premise of ensuring the frequency offset estimation range, which improves the SNR of sequences including the phase information. And then it calculates the phase angles for each order difference of biased auto-correlation signals and makes a weighted mean, which overcomes the difficulty in meeting approximate conditions for Fitz method under low SNR circumstance. The simulation results show that the proposed method can achieve high accuracy estimation without reducing the estimation range under extremely low SNR circumstance, and the estimation precision meets the tolerance to residual frequency offset for the receiving system under the standard of IEEE 802.15.4k.