The ejector-powered engine simulator (EPES) system is an important piece of equipment in conducting an influence test of the intake and jet flow in low-speed wind tunnels. In this work, through the analysis of the structure and principle of EPES, three parts of the internal flow force were obtained, namely, the additional resistance before the inlet, the internal flow force in the inlet and the thrust produced by the ejector. On the assumption of one-dimensional isentropic adiabatic flow, the theoretical formulae for calculating the forces were derived according to the measured total pressure, static pressure and total temperature of the internal flow section. Subsequently, a calibration tank was used to calibrate the EPES system. On the basis of the characteristics of the EPES system, the process and method of its calibration were designed in detail, and the model installation interface of the calibration tank was reformed. By applying this method, the repeatability accuracy of the inlet flow rate calibration coefficient was less than 0.05%, whereas that of the exhaust flow rate and velocity was less than 0.1%. Upon the application of the calibration coefficients to the correction of the wind tunnel experiment data, the results showed good agreement with the numerical simulation results in terms of regularity and magnitude before stall, which validates the reasonableness and feasibility of the calibration method. Analysis of the calibration data also demonstrated the consistency in the variation law and trend between the theoretical calculation and actual measurement of internal flow force, further reflecting the rationality and feasibility of the theoretical calculation. Nevertheless, the numerical difference was large and further widened with a higher ejection flow rate mainly because of the accuracy of flow measurement and the inhomogeneity of internal flow. The thrust deflection angle of EPES is an important factor in correcting this issue. In particular, the thrust deflection angle becomes larger with small ejection flow and becomes smaller with an increase in flow rate, essentially exhibiting a general change of less than 10°.