A horizontal jet-mechanical combined rock breaking test device and method. The device includes a horizontal base. One end of the horizontal base is provided with a multi-mode cutter head. A jet-mechanical combined cutter is provided on the multi-mode cutter head. The other end of the horizontal base is provided with a surrounding rock stress simulation bin for loading a rock sample. The multi-mode cutter head is connected to a driving mechanism, and the multi-mode cutter head is configured to advance and rotate horizontally along the horizontal base under the action of the driving mechanism, so that the jet-mechanical combined cutter is capable of acting on the rock sample.