Control method of an auxiliary braking system of an agricultural vehicle comprising an internal combustion engine (E) operatively associated with at least one vehicle wheel (W), comprising a PTO to drive a tool (Tool) in rotation, wherein the system of auxiliary braking comprises at least one engine brake device (F, SCF, R, D), the method comprising the following steps in succession: (Step 1) acquisition of a first value of a resistant torque applied to the PTO, (Step 2) calculation of a second resistant torque value function inversely proportional to the first resistant torque value, to be generated by said auxiliary braking system, (Step 3) selection and activation of the engine brake device according to said second calculated resistant torque value.