Abstract: A new type of coherent THz light source is being built in Shanghai Institute of Applied Physics. It will use the femto-second electron beam passing through a variable elliptically polarized undulator to produce the high bright THz radiation with various linear, elliptical or circular polarization. The gap-fixed variable elliptically polarized undulator EPU10.0 was built and its magnetic fields were measured and optimized. The details of the EPU10.0 control and magnetic measurement is presented in the paper.