Magnetic springs are widely investigated as energy converters alternative and energy harvesters by converting mechanical vibrations into electricity at low frequencies. In this article the design methodology of novel magnetic spring is proposed to improve the dynamic performance and electrical power. The magnetic spring system is composed by floating magnet and two cylindrical neodymium fixed magnets located on the top and bottom within the cylindrical casing. The external magnets repel the middle (floating) magnet that causes the spring force between them. The advanced magnetic spring system includes a new case with two interior chambers positioned on upper and bottom around the fixed permanent magnets. The chambers are filled with magnetorheological fluid that ensure the higher electrical power in comparison to previous invention in absence of magnetorheological fluid. The general purpose of the realized energy harvester is to provide a novel design of magnetic system with magnetorheological fluid that has the advantages to achieve electrical power in a larger range of frequencies. Additionally, measurements of the displacements and magnetic flux densities have been conducted within a dedicated experimental setup to validate the prototype and its electrical performance.