As the main controller in the motion control system of three dimensional (3-D) mobile platform,the Grating feedback signals were disposed,and the corresponding instructions were sent in real time by the STM32 Micro processor.Aiming at the defect that,the digit and the count way in Micro processor was not applicable for the 3-D mobile platforms with a large stroke,a method to expand the count digit and way sensing was given,how to choose a reasonable query cycle was discussed,and extending grating measuring length was extended,which realized the application in platform.Experiments prove that this method counts accurately,has high reliability,and can meet the processing demand of the grating signal.