A helical undulator introduced by National Synchrotron Radiation Laboratory was reformed and its performance was researched. The helical undulator was installed at the linear part of a storage ring to produce coherent synchrotron radiation with high flux and circular polarization and to provide a VUV beamline. Based on improved source parameters, the polarization radiation characteristics of the helical undulator were analyzed and the power density distribution radiated by the helical undulator, thermal distortion and the slope error on the premirror surface were calculated with ANSYS. The results show that the maximal thermal distortion of the premirror is less than 2 μm in a photon energy range desired by a burning experiment and the maximal slope error is about 1 μrad. Obtained results satisfy the requirement of SR experiment very well.