Satellite-ground time transfer and inter-satellite time transfer have been widely used in such scientific research fields as the generation and maintenance of international general time-scale and satellite navigation system. The microwave two-way time transfer, satellite common view and radar auxiliary satellite-ground time transfer are discussed. Based on the time transfer system models and principles from the above-mentioned methods, the timing chart and derived the mathematical expression of clock difference are given. A significant conclusion is obtained by comparison with the main error source from these methods.