The invention, which belongs to the technical field of vehicle quality detecting, provides a bogie sub-frame detecting tool and a method thereof. The detecting tool comprises a supporting bottom plateprovided with two supporting units I and two supporting units II. The detection method includes: four adapters are arranged on four supporting platforms correspondingly in a sub frame; a support unitIII is supported at the middle part for supporting a sub-frame U-shaped end. Whether the four adapters match the four supporting platforms completely is determined to determine whether the sub-frameforming product is qualified. The supporting units I are fixed on the support bottom plate; an adjustment mechanism for adjusting the distance and angles between the two supporting units I and the twosupporting units II is arranged between the two supporting units II and the support bottom plate so as to simulate the actual state of the sub-frame unit during the bogie forming and application processes. The production quality of the sub-frame unit is guaranteed; a problem that the basic measurement method can not measure the related data is solved; a problem that the product is disqualified because of error accumulation of the traditional measurement method is solved; and the work efficiency and economic benefits are improved.