The China Spallation Neutron Source phase II (CNSS-II) is upgraded with superconducting cavity technology, which uses 648 MHz 6-cell superconducting cavity module in the energy range of 165-300 MeV. Three 6-cell superconducting cavities are integrated in each module. The superconducting cavity works in pulse mode. To ensure that the frequency of the superconducting cavity meets the operation requirements at 2 K, each superconducting cavity needs a set of low-temperature tuner to precisely adjust and control its frequency. According to the structure and operation characteristics of the 648 MHz 6-cell superconducting cavity, a low-temperature tuner is designed. The frequency offset of the superconducting cavity is compensated by a fast-slow combination mechanism. The basic performance of the tuner and the dynamic Lorentz detuning of the superconducting cavity in pulse mode are analyzed.