In this study, the evolution of extension twinning in extrusion-shear deformed ZC61 alloy, including nucleation, growth, and detwinning, during two-step interrupted dynamic compression along the extrusion direction (ED) and extrusion radial direction (ERD) was investigated using quasi-in-situ electron backscatter diffraction (EBSD) observation. The results show that numerous extension twins, <a> dislocation slips and <c+a> dislocation slips are activated and some detwinning behavior is observed after the second indirect reverse loading step. Most of the activated extension twins satisfy Schmid's law; however, some low-Schmid factor twins form during the compression processes along both the ED and ERD due to strain accommodation among adjacent twins. The slip trace method can determine specific slip systems initiated during the compression process based on the best match between the Schmid factor calculated using the Euler angle and the observed slip trace.