The small thrust measurement research is performed to address the problem of small thrust values， difficult testing and inaccuracy of measurement generated by electric thrusters on micro-nano-satellites. The mechanism model of 5~100 mN torsional pendulum thrust frame sensing unit is established by using the principle of leverage and torque balance. The deflection performance under different thrusts is studied， and the linear relationship between deflection displacement and thrust is derived. The ANSYS simulation software is used to simulate the sensing unit and obtain the displacement clouds under different parameter conditions. The theoretical data and the simulation results are compared and analyzed， and the result shows that the structural design of the sensing unit is reasonable.