<p>This study examines the natural frequency and frame vibration pattern of the tractor vibration system, with an agricultural wheeled tractor serving as the research object. Theoretical natural frequencies of vertical and pitching vibrations for a planar vibration model of a tractor with two-degree-of-freedom biaxial rigid suspension were derived, and experiments were conducted to test the vibration characteristics and natural frequency of a tractor traveling at moderate speeds on a Class D road surface. Modal analysis of the structure of the tractor frame was then carried out using ANSYS Workbench software. The findings indicate that the vertical and pitching vibrations of the fuselage are interrelated, with the natural frequency being contingent upon the mass of the tractor, the rotational inertia, the stiffness coefficients of the front and rear tires, and the axle spacing of the front and rear axles. The vertical vibration intrinsic frequency is mainly affected by the “mass + front and rear tire stiffness”, the pitch vibration intrinsic frequency is determined by the “moment of inertia + front and rear wheel stiffness + wheelbase” together. The natural frequency of the tractor fuselage resulting from engine operation is predominantly concentrated within the 0–40&#xa0;Hz range. The natural frequency of the tractor vibration system is primarily concentrated within the range of 3 to 5&#xa0;Hz. The peak natural frequency of the front axle is markedly higher than that of the rear axle, and the peak natural frequency in the pitch direction at the center of mass of the fuselage is higher than the natural frequency of the vertical vibration. The vibration amplitude in this frequency range can be reduced through frame optimization (e.g. optimizing the front and rear axle wheelbase and damping matching for the coupling characteristics of pitching vibration), which can directly improve comfort. The intrinsic vibration pattern of the tractor frame is predominantly a modal vibration pattern, characterised by overall vibration. The first, second and third orders represent first-order bending vibration patterns, while the fourth order indicates localised vibration. The fifth and sixth orders correspond to second-order bending vibration patterns. There is a “directional weak zone” in the overall stiffness distribution of the tractor frame, and the bending stiffness along the length direction of the frame presents “multi-region synergistic insufficiency”. It is necessary to eliminate the multi-order coupling basis of the first-order bending vibration through the “whole-area stiffness balanced design”, such as optimizing the spacing of the cross beams and adjusting the gradient distribution of the cross-section moment of inertia along the length direction. The intrinsic frequency of second-order bending vibration can be enhanced through “end stiffness enhancement design”, such as adding variable cross-section longitudinal beams at the rear of the frame and adopting bionic bending-resistant structures, to avoid the attenuation of dynamic performance under high-frequency working conditions.</p>