A parameter tuning method of an unknown proportional-integral-derivative (PID) controller is provided. The unknown PID controller is replaced with the control algorithm of the generic controller to perform the optimal parameter tuning to obtain the target parameter of the generic controller. The unknown PID controller is set with a first parameter group so that the corresponding input signal, the control signal and the output signal are measured for performing the parameter identification procedure of the generic controller to obtain a second parameter group of the generic controller. When the second parameter group is not within a specification range of the target parameter, the first parameter is re-calculated and modified by a direct search method in accordance with the difference between the second parameter group and the target parameter for setting the unknown PID controller again, and then the input signal, the control signal and the output signal are measured again.