The invention discloses a rock tri-axial test crack extension observation device based on the electrical capacitance tomography technique.The observation device comprises a tri-axial test device and an electrical capacitance tomography system, wherein the tri-axial test device comprises a tri-axial chamber for placing a rock sample, a rubber sleeve for wrapping the rock sample, and a double-layer rubber ring surrounding the rubber sleeve tightly, the electrical capacitance tomography system comprises annular electrodes located between the two layers of the double-layer rubber ring and distributed in the ring direction of the double-layer rubber ring at intervals, a data acquisition module used for measuring the real-time capacitance between any two annular electrodes and converting the real-time capacitance into a capacitance digital signal, and an imaging module used for converting the capacitance digital signal into an image to be displayed, the imaging module sends out a detection digital signal, and the detection digital signal is converted into a detection electric signal through the data acquisition module to provide power for the annular electrodes.Crack extension of rock is observed with the electrical capacitance tomography technique, and visualization of the crack extension process can be achieved.