The utility model discloses a truck-mounted drilling rig and mast mechanism, mast mechanism includes fixed mast, first stay cord and unit head, still including the activity mast, its top is provided with the catead, and its bottom is stretched into in the said fixed mast, the hydro-cylinder, its stiff end with said fixed mast rigid coupling, its expansion end with said catead rigid coupling, the one end of first stay cord with said fixed mast rigid coupling, other end pile warp is located behind the first assembly pulley of catead with said unit head is connected. This mast mechanism is the telescopic structure, and its activity mast can be followed the fixed mast and reciprocated, compares with current fixed mast mechanism, has increased playing to pull out the stroke and pressurizeing the stroke of unit head for the truck-mounted drilling rig can be suitable for the drilling rod more than 9 meters, and so as to satisfies the drilling depth demand that increases day by day, in addition, because mast mechanism is scalable, when the transition transportation, activity mast relatively fixed mast is in the requirement that the withdrawal state can satisfy the transportation rules