The invention discloses a gantry mechanical arm for automatic assembling of an internal combustion engine. The gantry mechanical arm for automatic assembling of the internal combustion engine comprises a portal frame, an X-axis mechanism, a Y-axis mechanism, a Z-axis mechanism and a clamping jaw mechanism, wherein the X-axis mechanism is arranged on the portal frame; the Y-axis mechanism is arranged on the X-axis mechanism; the Z-axis mechanism is arranged on the Y-axis mechanism; the clamping jaw mechanism is fixed to the Z-axis mechanism and comprises a clamping jaw base, a clamping jaw rotary seat, a rotary shaft, a rotary motor, a clamping jaw fixing plate, a clamping jaw air cylinder, a linkage gear, clamping jaws and spring pressing posts; the clamping jaw base, the rotary shaft and the rotary motor are fixed to the Z-axis mechanism; the rotary motor drives the rotary shaft; the clamping jaw rotary seat is fixed to the rotary shaft; the clamping jaw fixing plate is fixed to the clamping jaw rotary seat; the clamping jaw air cylinder, the linkage gear, the clamping jaws and the spring pressing posts are arranged on the clamping jaw fixing plate; and the clamping jaw air cylinder drives the clamping jaws through the linkage gear. In this way, the gantry mechanical arm can convey the internal combustion engine to an appointed position and is suitable for internal combustion engines of various models.