According to the Jacobson's Molecular Free Length Theory in liquids and the relationship between the ultrasonic velocity and the molecular free length in organic liquids, a relationship between the parameter of every component of the mixture and the nonlinear acoustic parameter B/A and the characteristics of the ultrasonic velocity in the mixtures made up of multi kinds of organic liquids is derived. When the parameter of every component (the density, the ultrasonic velocity, the free length, the isothermal compressibility coefficient, the isotonic expandibility coefficient、the nonlinear acoustic parameter B/A, ect.) is given, the characteristics of the ultrasonic velocity and the nonlinear acoustic parameter B/A of the mixtures can be calculated. The calculated results are in agreement with the experimental results.