After analyzing 154 271 signals of instantaneous velocity of flow collected from 173 groups measured, probability model for instantaneous flow spectrum is established based on stochastic analysis theory, fluid dynamics and stochastic signal analysis theory. According to the research and discussion of distribution law of circular frequency and initial phase, it is found that circular frequency satisfies uniform distribution , initial phasealso satisfies(0,2π)uniform distribution. The research lays a solid foundation for the follow-up study of re-construction of random particle probability model and deduction of random acceleration spectrum and fluctuation pressure spectrum.