Magnetic resonance imaging (MRI) is increasingly used for robotic needle-based clinical diagnosis and therapy due to its high resolution and high soft tissue contrast. A needle driver therefore becomes an essential part of these MRI -guided robotic systems to perform in-bore needle placement under continuous MR imaging. However, existing actuator designs for needle drivers are constrained by the high magnetic field and the bore size of the MR scanner, and most lack high- performance capability, especially in terms of large output force and long stroke. In this work, we introduce an MR-safe concentric bellows linear actuator (CBLA) with improved performance over the existing designs. This soft actuator achieves 45N output force, 77 % stroke-length ratio for the bellows, and around 250% stroke-diameter ratio. A mathematical model was built to characterize the behavior of the actuator to provide guidance for the actuator design. The fabrication and experimental evaluation of the actuator are also presented. The results demonstrate the effectiveness of the CBLA design and the strong potential of its application in MR-guided surgical procedures.