A method for guaranteeing data transmission and a communications device are provided. A method for guaranteeing data transmission applied to a first communications device includes: obtaining first information; and determining, based on the first information, a differentiated services code point DSCP corresponding to an Internet protocol security tunnel IPsec tunnel; where the first information includes at least one of the following: quality of service QoS information of a second network tunnel, data type information, a mapping relationship between differentiated service code points DSCPs and QoS information, a first QoS information requirement, and a first DSCP.