The b -value is closely related to the crustal stress. By studying the b -value trends of regions before large earthquakes, we can obtain the evolution of crustal stress, which is generally considered useful earthquake precursor information. A quake is closely related to fault tectonic movement. In the past, the traditional b -value calculation method did not consider the spatial distribution of the fault. Here, we propose a new algorithm based on the fault’s buffer area, combining the spatial distribution and trend of the fault. This algorithm can accurately calculate the b -value changes for a specific fault using the fault as the basic independent unit. This method provides a useful reference for earthquake hazard judgment. Based on this algorithm, we use a total of 428,963 earthquakes from 1980 to 2013 after removal of the aftershock sequences and combine these quakes with the active fault distribution data to study the b -value changes before major earthquakes in western Sichuan. The results show that the b -values of the Fubianhe fault, Longmenshan main fault, and Shuangshi–Pengguan fault in western Sichuan decreased for at least 2 years before the earthquakes. The Longmenshan main fault area began to decrease slowly 5 years before the Wenchuan Ms8.0 earthquake and sharply decreased in the year before the quake, which reflects a significant change in crustal stress during this time. This conclusion and the method are available for long-term earthquake hazard assessment as a reference.