The invention discloses a roadside seal device for retaining roadways along goaf. The roadside seal device comprises seal mechanisms, support mechanisms and lifting mechanisms. The seal mechanisms seal a gateway, the support mechanisms provide support force for the seal mechanisms, and the lifting mechanisms drive the seal mechanisms and the support mechanisms to rise and fall. The seal mechanisms comprise flexible cord fabric. The support mechanisms comprise carriers and support blocks. The top ends of the flexible cord fabric surround and wrap the support blocks and are in attached contact with a roadway roof plate, and the bottom ends of the flexible cord fabric are in attached contact with a roadway bottom plate. The invention further discloses an application method of the roadside seal device for the retaining roadways along goaf. According to the method, heading directional blasting of an immediate roof is performed before coal face mining, and multiple groups of connected seal mechanisms, support mechanisms and lifting mechanisms are arranged in the length direction of the gateway in sequence. It is guaranteed that the gateway is completely isolated from the goaf, air flowing through is prevented, the roadway seal effect is good, all related parts of the seal mechanisms and the support mechanisms can be recovered, and therefore seal cost is saved.