Abstract: Based on Pg wave velocity inversion and earthquake relocation, this paper analyzes the seismic spatial correlation length of Pg wave velocity in the transition zone of Hetao seismic belt by using the single link cluster (SLC) algorithm and power-law fitting. The inversion results of Pg velocity show that the lateral variation of Pg wave velocity images is dependent on the structure, the velocity of Pg wave is positively correlated to the thickness of the crust, and two transitional zones of Pg wave velocity are formed in Baotou-Xishanzui bulge and Daihai sag. Furthermore, the seismic spatial correlation lengths of the two velocity transition zones are calculated based on the relocation data. The results show that the power-law fitting curve exhibits a trend growth to a certain degree, suggesting that the stress level of the above two Pg wave velocity transitional zones have been enhanced since 2008, and regional faults are likely to be entered into the stage of coordination. In addition, the Pg wave velocity transitional zone is usually a strong area of crustal movement， therefore it is deduced that the two zones will become the favorable place for occurrence of moderate earthquakes in future. On the condition of effective control of the positioning error, seismic relocation can reduce the discrete form and improve the calculation accuracy of seismic spatial correlation length.