As the demand of marine resources is continuously growing, more and more people are focusing on the study of underwater towed system for marine survey, in which mastering and predicting the dynamic characteristic of the system is the key problem. Based on the parameters of a certain underwater system, combined with the lumped mass method, the underwater cable-body 3D motion mathematical model has been established by OrcaFlex, in which the variation of the tension of the towed cable and the variation of the depth of the towed body have been given and the effect of the towed speed on critical radius in the process of the 360° rotary motion has been discussed. The results show a good agreement with previous research results. At the same time, we also make a research on the effects of the change of the hydrodynamic coefficients and the parameters of towed cable on variation of the tension and towed depth. These studies can provide a basis for the selection of cable in the system.