This chapter presents the establishment of a nonlinear mathematical model for hydraulic impact mechanisms. The fundamental equations governing the piston, valve spool, high-pressure accumulator, and high-pressure oil flow balance under various operating states are first summarized. Formulas for leakage and pressure differential calculations during different motion phases of the piston and valve are then systematically provided. The return oil motion under the condition of a return accumulator is analyzed, and a dynamic model of the return oil system is developed. Finally, the mathematical models corresponding to 12 typical working states of the hydraulic impact mechanism are summarized in tabular form.