Abstract: Using earthquake catalogue of China Earthquake Networks Center from 1970 to 2017, the b values of the north edge of Ordos block were scanned by the maximum likelihood method after evaluating completeness and deleting aftershocks. The results show that the low b value region of the north margin of Ordos block is mainly distributed in the Alxa area and the Shanxi-Hebei-Inner Mongolia area, and the spatial distribution of low b value region changes with time to a certain extent. Meanwhile, there is a certain corresponding between the low b value region and the location of moderate-strong earthquakes. The retrospective test of 14 earthquake cases shows that ten earthquakes occurred in the low b value regions and their edges, and four earthquakes occurred beyond the low b value region. And then Molchan chart method and R value scoring method were used to test the prediction efficiency of b value spatio-temporal scanning. Taking the area below the threshold as the predicted area, the missing rate, the hit rate and the spatio-temporal occupation rate of different thresholds were calculated. The result of Molchan chart test shows that the prediction efficiency of spatio-temporal scanning is better than that of Poisson distribution, the probability gain of the optimal result reached 2, and the significance level reached 0.25. The R value curve shows that the highest R value is 0.28 at the threshold of 0.8 with 97.5% confidence level.