Proton beam therapy is an emerging radiotherapy technology that delivers most radiation doses to the Bragg peak region while sparing healthy tissues. However, uncertainties in proton beam range may lead to insufficient dosage and compromised therapeutic efficacy, making range verification in routine Quality Assurance (QA) crucial. Magnetic Resonance Imaging (MRI) enables direct visualization of proton dose deposition in water phantom. In this study, a cylindrical Halbach array was designed for implementation in a portable low-field MRI system dedicated to proton range verification. To address the space constraints when using MRI as QA equipment in proton therapy systems, the array configuration was optimized for minimal size and mass, while maintaining the field strength and homogeneity required for high-quality imaging. As the accuracy of range verification in proton therapy may be compromised by magnetic deflection effects induced by Halbach arrays, the energy-dependent beam trajectory perturbations with magnetic field in MRI was investigated using Monte Carlo simulations with the TOPAS code. At a low magnetic field of 0.25 T, the 240 MeV proton beam exhibited significant deflection along with range retraction and lateral deflection, necessitating correction during range verification.