The Runge-Kutta ray tracing method is introduced firstly,which is used as a basic ray tracing method for the medium of the discretionary refraction index distribution. Then towards the three-dimensional fluid field with the characteristic of discrete refraction index distribution,a numerical computation method of any point′s refraction index and gradient is proposed. During the process of ray tracing,an idea and method that tracing step distance is self-adaptive according to local refraction index gradient and grid geometry scale are carried out,which can improve the tracing precision and efficiency. The results show that the method has high precision and efficiency for the ray tracing of three-dimensional space with the discrete refraction index distribution.