In order to reveal the influence of van der Waals' force on Lennard-Jones fluid viscoelasticity from the micro scale, equilibrium molecular simulation method was used to research the liquid-solid coexistence Lennard-Jones fluid of 30 conditions in the range of ρ*=0.85-1.0 and T*=0.6-1.5 in this study. First, the viscosity of argon is simulated by this model and the results are consistent with the experimental value of the error within 6.69%, which verifies the expansibility of the model to the real substance. Then the liquid-solid coexistence Lennard-Jones fluids simulated, and the accuracy of the simulation in this range is verified by comparing the simulation viscosities with the literature values within the error of 5.16%. Finally, the variation of both static viscoelasticity (viscosity η*, high-frequency shear modulus G∞*) and dynamic viscoelasticity (storage modulus G'*, loss modulus G"*) were observed from the external factors (temperature and density) and internal factors (Lennard-Jones potential parameters, ε and σ), in addition, the microscopic mechanism of van der Waals' force effect on the viscoelasticity was elaborated as well. The results show that both density increment and temperature decrement lead to the increase of the static viscoelasticity (η*, G∞*), meanwhile, G'* and G"* in the high frequency region were also increased by the increment of temperature and density, which suggests the enhancement of the viscosity and elasticity. Furthermore, when Lennard-Jones potential parameter, ε or σ increases, the Lennard-Jones fluid tends to be more solidified, this enhances both static and dynamic viscoelasticity and provides a guide to improve the viscoelastic properties of monatomic material in the engineering applications.