The advantage of building a conductive network on the surface layer of a flexible substrate is that it has less impact on the elastic recovery properties of the substrate, which is particularly important for flexible strain sensors. However, the facile construction of robust conductive layers on the surface of flexible polymers remains a challenge. Herein, a method for constructing robust conductive layers on the surface of thermoplastic polymers was developed by immersing thermoplastic polymers in a solvent/conductive filler dispersion with the assistance of ultrasound. The solubility of the solvent in the flexible polymer and ultrasonic field are key to the preparation of the conductive layer. This method has the advantages of fast preparation and robustness of the conductive layer and can be applied to thermoplastic polymers of different polarities as well as different types of conductive fillers. Based on this method, a flexible strain sensor with a robust carbon black conductive layer on the styrene–butadiene–styrene block copolymer was prepared in as short as 2 s. The advantages of a broad strain detection range (0.1% to 400%) and robust cyclability of the sensors were exhibited. The sensors can be used for human motion monitoring as well as solvent detection.