Abstract: Using preferred plane theory,this paper developed a new analytical method for the preferred planes which control the urban shallow tunnel stability,taking the subway in Nanjing as an example.The new analytical method categorizes the soils into different layers and determines the preferred layers for the stability analysis of buildings in the soils.The results of the study on Nanjing subway are：① the regional preferred faults of Nanjing are Nanjing-Shuhu fault and Dinghuaimen-Gulou fault which control the tunnels stability in Gulou and Xiaohongshan；②the most favorable and mechanically stable layer for Nanjing subway is Tertiary hard clay layer(Ⅲ1), the next is the Tertiary soft clay layer(III2)；the most unfavorable layer for subway is Quaternary soft clay layer(II2)and loose layer(II4)；③the elevation of bottom of Nanjing subway tunnels is suggested to be at -5 ~-15m..