The safety factor variation laws of tunnel of HongdaoJiaonan Intercity Railway in Qingdao laterally crossing adjacent pile foundations of bridges in uppersoft and lowerhard strata are studied by finite element strength reduction method. The results show that: 1) In horizontal direction, the safety factor increases with the increase of horizontal clear distance; and then the safety factor becomes stable when the clear distance reaches the value of tunnel diameter. 2) In vertical direction, with the increasing of the buried depth, the safety factor increases firstly and then decreases. 3) The safety factor of the tunnel would be reduced and the buried depth of the tunnel for reaching maximum safety factor would be increased due to the existence of pile foundations. The interactions between metro tunnel and pile foundations of bridge at different relative positions are as follows: 1) Generally, the vertical interacting scope and degree is larger than the horizontal ones. 2) The tunnel would be affected only if the buried depth of the tunnel is smaller than the buried depth of the pile foundation. 3) The tunnel and the pile foundations would both be affected when the tunnel is under the pile foundation bottom; and the interaction effect reaches peak when the tunnel is right under the pile foundation bottom. The universal sectioning sketch of interacting factor, isogram 0.8 as the boundary of strong interacting zone and weak interacting zone and isogram 1.0 as the boundary of weak interacting zone and noninteracting zone, is given by interpolation method based on polynomial spline method, multiple quadratic surface fitting method and the stress and numerical simulation based deformation characteristics of pile foundation. Finally, the practicability of the study results are testified by comparing sectioning of interacting zones and monitoring results.