Abstract: A hexagonal castellated beam with cracks was designed and fabricated, of which the mechanical properties, including the location and length of cracks at the weld, were examined under different parameters. The experimental also devised a method to calculate the flexural resistance of hexagonal castellated beams damaged by weld cracks and further assessed how the strain and deflection of the test beam with crack varied as the parameters altered. The findings indicate that the failure mode of the hexagonal castellated beam with weld crack damage is the buckling failure with structural hole angles initiating to yield. Once a specific amount of hole angle yield has been achieved, the castellated beam will start experiencing damage. During the repeated loading phase, the effect of crack length on the angular strain of the hole near the weld is within ±20 με and the deflection of the critical section position is within 0.05 mm. The flexural stiffness of the castellated beam with the web weld crack was calculated by equivalent stiffness method and the effect of web weld crack on the mid-span deflection was investigated. By incorporating the deflection amplification factor, the formula for determining the mid-span deflection of the castellated beam under a particular load is obtained. By calculating the mid-span deflection of castellated beams with various formulas and contrasting the outcomes, we can observe that there is a significant disparity of around 50% between the test results and the results from the Japanese standard estimation approach as well as the simplified method. The proposed results are in accordance with the experimental results, and most of the discrepancy is within 10%.