  The specific energy loss (dE/dx) resolution is critical for particle
identification and separation in TPC. In connection with the R&D for the TPC at
the Circular Electron Positron Collider (CEPC), we find the laser ionisation
obeys the Gaussian distribution theoretically and experimentally. Based on
this, we develop a novel method using a 266 nm UV laser to measure the dE/dx
resolution. Comparing to the traditional methods with particle beams or cosmic
rays, the novel method with 266 nm UV laser is more convenient and flexible. We
verify the method's feasibility with a TPC prototype and extrapolate the dE/dx
resolution to the CEPC TPC. The estimation of the dE/dx resolution is without
the magnetic field.
