Based on the analyses of all the ways to block gasoline supply in idle running, the authors made the choice of electric-magnetic gasoline saving appliance to block gasoline supply in idle running and did the research and manufacture on it , according to the vehicle application characteristics for transporting wood in forest areas under a heavy load condition to go down mountains in the way of forced idle running constantly. The article briefly introduces the construction, arrangement and the results of loading tests of the gasoline saving appliance and also shows its reliability of gasoline-saving and ability to guarantee vehicles safe to go down the mountains.