Mechanical products usually consist of many components and parts.A mechanical part always shows variations in its ge-ometrical characteristics because of uncertainties in manufacturing processes.Quality is often reflected by how well tolerances and func-tional requirements are being achieved by the manufacturing processes in the final product.The appropriate allocation of tolerances among the different parts of an assembly is the fundamental tool to ensure assemblies that work correctly at a lower cost.Therefore,an efflicient tolerance analysis method must be applied.But existing models are often based on points which are limited and difficult,and the impact of the tilted tolerance zone along the direction of analysis isnt considered in many models.The paper proposes the unification of modified Jacobian matrix and modified torsor along the direction of analysis.A new method to get the stack-up path is proposed.The calculation of percentage contribution to assembly error from impact factors is presented.This approach has been illustrated by a numerical example.