A retaining device for a linear transmission system comprises: a sliding block, a rail clamp and a spring fastener The sliding block includes an upper retainer formed with a positioning hole. The rail clamp includes two retaining members and two elastic members, and the two retaining members are symmetrically arranged and are connected by the two elastic members. The two elastic members can adjust the interval between the two retaining members. The spring fastener is pivotally arranged between the two retaining members of the rail clamp and is provided with two elastic retaining portions to be engaged in the positioning hole of the upper retainer. After the rail clamp is installed in the sliding block, the two elastic members of the spring fastener will be engaged in the positioning hole, thus fixing the rail clamp in the sliding block.