The invention relates to a method for assuring the projection relationship between internal fields of span service field. Wherein, said method comprises: the inlet point of field receives the path information that loaded service level consultation SLA projection target information; combined with said SLA information and the SLA consultation strategy data of each node, assuring the inter-field SLA projection relationship of span service. The invention distributes different strategies on different fields, to improve the stability of service, and reduce the service cost.