A composite working implement is an agricultural machinery work tool that performs simultaneous cultivating and ridging. This may be critical in terms of structural strength and durability, because these implements are subjected to irregular, random, and high loads during work. In this research, a load reconstruction technique utilizing strain gage signals measured with field tests was used to derive load histories for a disk harrow mounted on the composite working implement. Additionally, the optimum points at which the strain response reaches maximum were found using the disk harrow finite-element analysis results. A method is proposed for reconstructing the load histories from measured strain data at a tractor velocity of 14~16 km/h, identifying the principal direction with the highest fatigue damage direction in the x-, y-, and z-axes, and projecting loads on the axis for conversion into an equivalent load.