Based on cyclic loading experiment of nine beams in the two groups, the relationship among the average web stress, initial diagonal crack width and shear capacity of RC beams was studied. Experimental result shows that the decline of the shear strength of the RC beams due to cyclic loading can be characterized by the width of diagonal crack. Under the same load amplitude, the larger crack width is, the greater the decline of the bearing capacity is. Besides, under the same number of cycles, the greater the load amplitude is, the larger the crack width is, the greater the decline in the bearing capacity of beams is. In addition, according to the experimental phenomena and the shear compression failure mechanism of beam, a formula for shear capacity after cyclic loading is derived for engineering use.