The invention provides a calculation method and device for product assembly deviation, and relates to the field of mechanical engineering. The method comprises the steps that according to tolerance information and assembly information of a product assembly prototype, and the positioning relations between geometrical features of parts of the prototype are determined; the theory constraint direction between the geometrical features is analyzed according to the positioning relations, and the actual constraint direction between the geometrical features under the constraint of positioning priority is analyzed; according to the actual constraint direction, a variable vector space is established; according to the variable vector space and the positioning relations between the geometrical features, assembly variation transfer lines corresponding to functional requirement information of the product assembly prototype and the constant directions of all the transfer lines are obtained; according to the assembly deviation transfer lines, whether the tolerance information and the assembly information are redundant or missing is judged, and a judgment result is obtained; according to model information, the tolerance information and the judgment result of the product assembly prototype, the magnitude of all deviation sources on the assembly deviation transfer lines is obtained, and the calculation of the assembly deviation is completed.