The modified gear involute equation under Cartesian coordinates system is established and the theory foundation is provided for parameterization modeling of modified gear. The character of drop arm thickened-tooth segment of steering gear is analyzed. The modified gear involute parameterization equation is established based on the CATIA and the method of parameterization three-dimensional modeling of the thickened tooth segment is detailed. Then the space analysis and motion simulation are implemented for the Digital Mock-Up of steering gear. This method is high modeling accuracy, it is very convenient and the accurate thickened-tooth segment models can be obtained quickly by changing the relevant parameters, which provide the basis for digital assembly and NC machining.