Abstract: Active fluids hold great potential for the development of new materials, but realizing this potential requires a quantitative understanding of the mechanical behavior that these fluids exhibit, especially the rheological behavior. The Brownian motion equation is extended to establish the kinematic model of self-propelled particles. The viscosity of active fluid is determined based on the reverse non-equilibrium molecular dynamics scheme. The effects of volume fraction, forward locomotion velocity and rotational diffusion coefficient of active particles on the rheological behavior of active fluid are investigated, and the formation mechanism of special rheological behavior of active fluid is determined. The results show that the rheological curve of the active fluid can be divided into viscosity reduction regime, transition regime and Newtonian regime. The higher the volume fraction of the active particle is, the more significant the non-Newtonian properties of the active fluid are. The forward locomotion of the active particles leads to the reduction of the viscosity of the active fluid in the low shear rate region. The coupling effect of forward locomotion and rotational locomotion leads to the non-monotonic change of the rheological curve in the moderate shear rate region, and the frequent rotational locomotion of the active particles leads to the inhibition of the non-Newtonian properties of the active fluids. The fluctuation of active particles makes the active fluids have special rheological behavior. The higher the volume fraction of active particles, the faster the forward locomotion velocity and the smaller the rotational diffusion coefficient are, the easier the active particles produce obvious fluctuations in the active fluid is. The fluctuation of active particles is obvious in the low shear rate region. With the increase of shear rate, the fluctuation of active particles is gradually weakened, and the aggregation structure of particles is constantly destroyed. Finally, the rheological behavior of the system is similar to that of the general passive fluids.