The diagnostic conclusions by three-ratio method cannot be directly applied to the construction of the basic probability assignment (BPA) function. In order to solve the problem, this paper proposes a calculation method for transformer fault BPA based on spatial interpolation. According to the basic principle of finite element method, the fault characteristic distribution of three-ratio codes in the spatial geometry is analyzed and the entire fault characteristic space is discretized, and finally the continuous BPA function of faults in the entire space is obtained using spatial interpolation. The method meets the requirements of BPA function, and its calculating result quantitatively reflects the probability of various faults. With examples calculation, the correctness and feasibility of this method in actual engineering projects are verified.