The work point should be controlled when open-loop Fiber Optic Gyroscope(FOG) is working,so a sin wave modulation signal with adjustable amplitude must be produced by digital method.First,mathematic description of FOG's modulation signal was analyzed,and then three methods for producing modulation signal with adjustable amplitude were discussed,which are direct wave table method,multiplicative amplitude adjusting method and two-level wave table method.Their advantage and disadvantage were analyzed.Experiment results of work point controlled by using different modulation signal producing method were given.In most cases,two-level wave table method is a good choice for producing modulation sin signal and can realize work point precise control in the condition of both limited storage resource and computing resource.