Permanent magnet array is a good candidate for providing the main magnetic field for a low-field portable MRI system. In this abstract, we present the design of a new yokeless permanent magnet array that generates a longitudinal magnetic field with a significant increase in field strength and in homogeneity compared to a traditional two-ring structure. It is compatible with existing RF coils thus the advancement in coil designs can be applied. The optimization was done based on genetic algorithm and a current model which shows much higher calculation efficiency than finite element method. The effectiveness of the optimization is validated by realistic simulations using COMSOL.