In the structural design of foundation pits in soft soil area, the overall stability plays a controlling role. For the pits which can’t meet the stability requirements, it is an effective solution to increase the penetration depth of supporting structure. However, this method works not obviously for the narrow pits with space effect. Taking a soft soil pit project for example, we account the coupling effect in the central of the narrow pit with double sliding surface, propose to increase the resistance torque of sliding using the passive earth pressure of the coupling surface, which will reduce the penetration depth of supporting structure effectively. This method provides a reference for narrow pit design in harbor engineering.