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A 2G high-temperature-superconducting (HTS) undulator with cryogen-free cooling is under development at the National Synchrotron Radiation Research Center (NSRRC). The design of the superconducting undulator requires the application of 2G high-temperature-superconductor tape in place of the traditional NbTi superconducting wires, and sets the working temperature to 25 K, which can be attained with a cryo-cooler. The superconducting undulator is thus no longer restricted by liquid helium. A special winding method with a resistive joint has been designed to allow modularization of the tape during winding, to facilitate that process. Moreover, the cooling channel has a sandwich structure designed for direct cooling of the resistive joint using the cryo-cooler. The cryogen-free concept, the design of the magnet core and the winding method of the HTS undulator are discussed in this paper along with measurement of the contact resistance and details of the testing of a seven pole prototype immersed in an LN2 bath.