Excel formula is widely used in data processing, and array formula improves Excel data processing ability to a new level. Because there are few reference materials for the Custom functions used in array formulas and the way to realize them is not clear, seldom the Custom functions are applied to array formulas. In this paper, the parameter transfer, parameter processing and function realization of Custom function for array formula are analyzed, and a general design method for Custom function used in array formula calculations is obtained.