A drilling mechanism of a coring drilling rig has a central rod, a fluid channel starting module, an outer barrel, an outer barrel unlocking module, a flow diverging module and a drill bit. The central rod penetrates the fluid channel starting module. The outer barrel unlocking module and an inner cavity of the flow diverging module from back to front. The fluid channel starting module is behind the outer barrel and is connected to the outer barrel unlocking module. The flow diverging module is in front of the outer barrel unlocking module, and a hydraulic motor is connected in front of the flow diverging module. The outer barrel has a driving section that is a rotor of the hydraulic motor. The outer wall of the outer barrel is fixedly connected to a centralizer, and the front end of the outer barrel is connected to the drill bit.