A parallel mechanism driven by 8 cables is presented to output vector force in an arbitrary constant direction for simulating rocket thrust in real time. By kinematics modeling and force analysis, dynamical equations of 8 cables in the presented mechanism are proposed. Based on constant-direction vector force output model, the cable tensions when outputting constant-direction vector force are calculated with the closed-form cable-force optimization theory. The rules of tension distribution when outputting vector forces with different values in various directions are proposed. In addition, constant-direction vector force is output by the experimental setup of 8-cable parallel mechanism, which demonstrates the rationality of the theoretical research.