In order to study the essential structural characteristics of monolayer MoS2 nanoribbons in natural state, and also the effects of the aspect ratio and the ambient temperature on the relaxation properties of the nanoribbons, the relaxation properties of monolayer MoS2 nanoribbons with different aspect ratios are simulated by molecular dynamics (MD) method based on REBO potential functions at different thermodynamic temperatures from 0.01 K to 1500 K. The energy curves and surface morphologies of monolayer MoS2 nanoribbon are obtained, and the dynamic equilibrium process of the monolayer MoS2 nanoribbons is also discussed in all the simulation process. The simulation results show that the monolayer MoS2 nanoribbons do not generate a fluctuation at the ideal temperature (0.01 K) for the reason that the kinetic energy of the monolayer MoS2 nanoribbons is almost zero and the vibration amplitude is small. However, a certain degree of fluctuations occurs at the edges and inside of the monolayer MoS2 nanoribbons at the room temperature or high temperature. The fluctuation height and the fluctuation degree also increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbon, even a high aspect ratio monolayer MoS2 nanoribbon exhibits a surface curved fluctuation, which is perpendicular to the surface of the MoS2 nanoribbons under high temperature condition. Finally, the influences of chirality on relaxation property under different temperature conditions are studied in this paper further, the results show that unlike the armchair structure, the zigzag monolayer MoS2 nanoribbons not only present a surface fluctuation, but also exhibit an obvious bending phenomenon along the width direction simultaneously. Like the armchair nanoribbons, the surface fluctuation height and the surface fluctuation degree of the zigzag nanoribbons also increase with increasing both the ambient temperature and the aspect ratio of the MoS2 nanoribbons. It is also observed that the armchair and zigzag monolayer MoS2 nanoribbons with a similar aspect ratio have a similar surface fluctuation degree at the same ambient temperature. Unlike the armchair nanoribbons, the bending phenomenon along the width direction of the zigzag nanoribbons is more significant, and the bending width and the bending degree increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbons. Although the bending degree of the zigzag nanoribbons becomes larger with the increase of temperature, the increasing rate of the bending degree will become smaller and smaller until the bending degree reaches a maximum value.