A vehicle idle-speed warning system and vehicle idle-speed detection method thereof are provided. It is non-invasive, that is, there is no need to change or modify any part of the constructing parts in the engine compartment. The present invention can be adopted stand-alone, or be applied to existing popular apparatuses, such as GPS, driving recorders, smart handheld devices, and vehicle electronic equipments. With the composing elements of a motion information module, an input module, an output module, a timer module and an information processing module, the present invention can accurately determine whether the vehicle under surveillance is in idle state and for how long it is in the idle state, and send an alarm signal automatically. With the implementation of the present invention, drivers can always be alarmed with the vehicle idling situations to prevent possible dangers or coming tickets due to the violation of traffic regulations in certain countries.