A device for dynamically generating numerical controller software of machine tool includes a software function-specification managing unit, a function-module storage unit and a function-module composing unit. The software function-specification managing unit is to produce a text description file describing function modules required by the numerical controller software, and then for performing specification analysis upon the text description file to generate a function module table including the function modules. The function-module storage unit is to memorize and manage the function modules, wherein the function-module storage unit confirms whether or not the function-module storage unit has been furnished with the f function modules required by the numerical controller software. The function-module composing unit is to retrieve the required function modules from the function-module storage unit, and further to set the connection among the function modules to compose the numerical controller software. In addition, a method for the device is also provided.