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The vibration transmitted to helicopter aircrew is the main health risk, specifically at low excitation frequencies. In this paper, to improve the vibration environment for aircrew, a seat suspension using a negative stiffness structure has been proposed. The main feature of the proposed system is the negative stiffness structure along with a traditional positive stiffness structure. Here, the dynamic model of the proposed system is derived and the design procedure for the seat suspension parameters is presented in order to mitigate the vibration transmitted to occupant and at the same time preserving payload capacity of the system. Then, the vibration transmitted to seat as well as the transmissibility of the proposed system at the steady state are evaluated using ISO-2631 and conventional criteria. To verify the validity of the simulation results, the reproduced signal of the cabin floor of the Bell-412 helicopter is applied. Results reveal that the isolation performance of the proposed system based on the negative stiffness structure is well-suited so that the values of root mean square and vibration dose value for seat’s vertical vibration in this structure are 0.0789 and 0.1254, respectively. It showed an almost 90% reduction of amplitude with respect to the cabin floor’s vibration. Also, according to the ISO-2631 standard, the level of comfort is improved from uncomfortable to not uncomfortable that represents the promotion of riding quality and improvement of vibration environment for aircrew. As well as results display that the frequency spectrum of the cabin floor and the seat are similar to each other and actually frequency modulation does not happen in the vibration transfer path between the cabin floor and over the seat.