Objective The online detection of thrombus and visualization are of great importance in the extracorporeal circulatory system. In this paper, a noninvasive and online detection method of thrombus in extracorporeal circulation based on electrical impedance tomography (EIT) is proposed.Methods Firstly, this paper investigates the effect of sensor size on the imaging performance through joint simulation. Secondly, a 16-copper electrode EIT sensor with a diameter of 20 mm is designed based on the simulation results, a cyclic flow experimental platform is built, and static and cyclic flow experiments are designed. Instead of thrombus, pig blood clots of 3-6 mm in size are placed in fresh pig blood samples and image using the Tikhonov regularization algorithm. The 3 mm and 5 mm clots are placed in the circulation system, and the images of the size and position of the clots in the sensor section are reconstructed by the EIT system and compared with the results taken by the high-speed camera.Results The simulation results show that the image correlation coefficients (IC) are greater than 0.06 for the sensor diameters of 20 mm and 30 mm when the target-to-sensor area ratio (AR) is not less than 0.01, and the imaging effect is well. The static imaging results show that the relative size coverage ratio is less than or equal to 0.1. The circulating flow experiments show that the normalized relative conductivity value changes of 80 and 200 are detected as the clots pass through the sensor, and the results show that the method is capable of detecting clots in the circulatory system.Conclusion The method has the advantages of real-time and noninvasive, and is expected to be applied to the detection of thrombus in vitro.