The invention relates to a level measuring apparatus and a method for a guide plate beam of a high-speed wire mill. The level measuring apparatus comprises a base, a ruler, a first slider, a second slider, a screw rod, a clamping sleeve, a fine adjusting nut, a specification bar and locking nuts, wherein a base keyway matching with a guide plate beam key is arranged on the lower surface of the base, the ruler with scales is vertically arranged on the base, the first slider and the second slider are arranged from bottom to top on the ruler and connected through the screw rod, scales and the clamping sleeve for clamping the cylindrical specification bar are arranged on the first slider, the fine adjusting nut is arranged on the second slider, the first slider and the second slider are located on the ruler through the locking nuts respectively, and the section diameter of the specification bar is the same with the specification of the roll pass of a rolling mill corresponding to the guide plate beam to be measured. Compared with the prior art, the apparatus and the method have the advantages that the structure is simple, the design is reasonable, the utilization is convenient, the measurement is rapid and accurate, the on-line installation accuracy for rolling guide plates at an inlet and an outlet is guaranteed effectively, stable operation of the production is guaranteed, and the cost is low.