The invention relates to the technical field of rotary drilling rig construction, in particular to a simple measuring method of verticality based on a prismatic-free total-station instrument. The simple measuring method of the verticality based on the prismatic-free total-station instrument comprises the steps of adjusting a self-contained vertical system of a drilling rig to 0, positioning a mounting point of an instrument, observing and adjusting offset. The risk of construction personnel climbing a mast on the drilling rig top for operation is avoided by a laser ranging method of the prismatic-free total-station instrument; deviation values in the front, back, left, right four directions are calculated by measuring horizontal distance and angles between the top and bottom of the mast onthe drilling rig top and the prismatic-free total-station instrument and matching with an angle difference between the top of the mast and a bottom corner angle; and the verticality of the mast is ensured by adjusting offset of the mast according to the deviation values in the four directions, so that an operating machinist in an operating room is further prevented from paying attention to the display of the verticality at all times. The simple measuring method of the verticality based on the prismatic-free total-station instrument solves unsafe factors of personnel operation, reduces the engineering quantity of large-scale treatment of the invading pile foundation on site, and indirectly increases the income of a project department.