The invention relates to a flatness measurement instrument which comprises a rack, wherein a height detection instrument and a linear motion structure for driving the height detection instrument to do linear motion are arranged on the rack, the flatness measurement instrument further comprises a locating structure arranged below the height detection instrument, the locating structure comprises a locating base for placing a to-be-measured workpiece and a rotating structure for driving the locating base to rotate, and the height detection instrument is driven by the linear motion structure to coincide with a rotation axis of the locating base and to be staggered with the rotation axis of the locating base. The measurement instrument is high in measurement accuracy, only the height measurement instrument needs to be well placed in the measurement process, the operation convenience in the measurement process is greatly improved, and the error caused by manual measurement is reduced.