Abstract: Lyotropic liquid crystal exhibits excellent biocompatibility, non-toxicity, biodegradability, optical anisotropy, and electromagnetic anisotropy. It has been widely used in the areas of biology, medical engineering and liquid crystal displaying, such as cell interaction, nerve stimulation transmission, fat absorption and intelligent drug transport. In this paper, the rotational viscosity of the lyotropic liquid crystal Sunset Yellow in the nematic phase at different temperature and solution concentration is measured by the rotating magnetic field method. Combined with the self-assembly process of lyotropic liquid crystal molecules, the variation of rotational viscosity of lyotropic liquid crystal in the nematic phase with temperature and solution concentration is analyzed theoretically. The results show that the rotational viscosity of the lyotropic liquid crystal is positively correlated with the square of the average length of the self-assembled columns, increasing with the increase of concentration, but decreasing exponentially with the increase of temperature. The empirical expression of rotational viscosity related to temperature and concentration of nematic lyotropic liquid crystal is constructed. The calculated results of the empirical expression are in good agreement with the experimental values, with the maximum error of 18.56\%. A new indirect method of obtaining the shear energy of lyotropic liquid crystal by using rotating rheometer is proposed. The shear energy of lyotropic liquid crystal increases with the increase of solution concentration, but it is hardly related with the change of temperature in the experimental range. The shear energy obtained in this paper is in good agreement with the results of Joshi et al. who used the X-ray detection method, with the maximum error of 3\%. The influence of length-diameter ratio of the columns on the rotational viscosity is investigated by using the variation of self-assembly capacity of liquid crystal molecules with temperature. The "one-step method" measurement is proposed, which greatly reduces the expenses and complexity of related experimental researches.