A method for the ultrajet diagnostics of products made of composite materials is presented. It makes it possible to simulate, in an accelerated regime, the fatigue tests of materials and to estimate the adhesion strength of the fiber-filler bond under alternating loads. A hydroerosion model of the diagnosed surface under the operation of sequentially conjugate ultrajet actions by various working hydraulic pressures of the ultrajet water applied is considered. The concentration of the functionally significant defects formed at a certain stage in the life cycle of the object of analysis, for example, in the surface layer of the diagnosed product, is taken as a general characteristic of the hydroerosion of the material. Relations that determine the kinetics of formation of an ultrajet hydrocavity on the surface of the object of analysis are obtained. The time spent by the ultrasound on the erosion destruction of the softened layer is determined, which is a new information-diagnostics criterion, since this time characterizes the correlation between the level of action on the surface layer of material and its physical and technological reaction to the ultrajet effect. Results of this research will help one not only to clarify the fracture laws of composites under unsteady loadings, but will also ensure the creation of composite materials with predetermined physical, mechanical, and chemical properties for specific application conditions.