Abstract: To investigate the performance of a Mach 8 scramjet, a free flight force measurement technique was developed in a high-enthalpy free piston driven shock tunnel FD-21. A nonlinear motion track method integrating the circular feature markers and image recognition was proposed. The scramjet model was suspended and released by a novel electric permanent magnet which controls the model accurately. A high-precision sequence control system was adopted to implement the time synchronization. To facilitate the tracking of the scramjet model, a trapezoidal porous circular array marker plate was designed and mounted on the outward surface of the model. The lamp-house illumination method was employed in the backlight system, in which the maximum power of the photosource is up to 300 W and the average brightness can reach 45000 Lux at one meter working distance. Based on the characteristics of circular array, a typical model tracking method using the image recognition technique was studied. An outlier elimination of the original displacement data was implemented using the nearest neighborhood algorithm. The noise-free acceleration was subsequently obtained by the second derivative of the filtered displacement. Data processing methods including the motion displacement extraction were discussed by two types of experiments involving a free-falling scramjet in the quiescent air and a free-flight scramjet without fuel injection at Mach 10. The force data of the engine model was obtained, and the precision of measurement technology was discussed. For the former, the measured gravity acceleration is consistent with the local true value with a deviation of only ±2%. For the latter, there exhibits a good repeatability with a deviation of 2.32% for the horizontal acceleration and 7.44% for the vertical acceleration. In the future research plan, the protection device of the electric permanent magnet will be further optimized. In addition, more measurement methods, such as triaxial accelerometers and stress wave balances, will be used to realize cross validation by comparing with the method based on multiple-circles tracking and backlight imaging, so as to verify the universality and applicability in the thrust resistance measurement of scramjets in high-enthalpy flows.