Abstract: A rotary slender beam in a pipe is a special structure in oil engineering. It not only produces random multi-directional collisions with the pipe’s inner wall, also couples with the inner and outer annular fluids. This results in a complex system of nonlinear fluid-structure interaction. Up till now, no research literature concerning this aspect is available. In the paper, in terms of the structure and mode of operation about a rotary slender beam in a pipe, equations of the structure dynamics, the fluid equation of continuity and momentum equation have been coupled. Thus the calculation formulas about the interface force and displacement, the congruent iterative format and convergence criterion have been derived. A three-dimensional numerical model and a computational method have been established for the dynamic analysis of the fluid-structure interaction of the rotary slender beam in the pipe. The numerical example shows that the proposed method is capable of describing the dynamic behavior of the fluid–structure interaction, and reveals the effect of the fluid–structure interaction of the rotary slender beam in a pipe with higher computational accuracy and efficiency. And the numerical results match well with those obtained by experiments. The present methodology provides a feasible computation model and research approach for the fluid–structure interaction of the string in oil drilling and production engineering.